Airplanes, Altitudes and Airspeeds
Introduction
Flying an airplane requires reference to several different airspeeds and altitudes. There are several important concepts to understand to successful operate your airplane
and to interact with air traffic controllers.
Abbreviations / Definitions
ATC: Air Traffic Control/Controller
Nautical mile (NM): 1 NM = 1.85 km = 1.15 statue miles, approximately anyway; more precisely, a nautical mile is defined as one minute of latitude - there are 60 minutes in one degree, so 1 degree of latitude equals exactly 60 nautical miles.
Knot: A measure of speed representing one nautical mile per hour.
IAS: Indicated Air Speed
TAS: True Air Speed
GS: Ground Speed
MSL: Mean Sea Level - altitude expressed in distance above Mean Sea Level
ASL: same as MSL, except the "A" stands for "above"
AGL: Above Ground Level - altitude expressed in distance above the ground
How Do We Measure and display Airspeed and Altitude?
Pitot Tube / System
The pitot tube is a device mounted outside the airplane to measure airspeed. It is aligned with the airstream so that it can measure the pressure of the air created by the air moving over the airplane's wings. In light aircraft the pitot tube is connected directly to the airspeed indicator; larger aircraft generally incorporate an air data computer and in
those cases the pitot tube is connected to that computer. Light aircraft will normally have one pitot tube, transport aircraft will have at least three.
Static Port / System
Static port(s) are mounted outside the aircraft to measure static air pressure. It is oriented so that it is unaffected by the changes in air pressure caused by the movement of the airplane through the air. In light aircraft, the static port(s) are connected directly to the altimeter, airspeed indicator, and vertical speed indicator. In larger aircraft, static ports are connected to the air data computer, which will calculate various pieces of information based on the static input. Small aircraft generally have one or two static ports (the second port allows the system to compensate for times when the airplane isn't in perfectly coordinated flight). Transport aircraft will normally have at least six static sources.
Airspeed indicator
The airspeed indicator compares pitot air pressure to static air pressure and displays the result as an "indicated airspeed". In airplanes with an air data computer, the airspeed indicator itself is fed airspeed data from the air data computer.
Altimeter
An instrument which measures outside air pressure from the static system and converts that pressure to an altitude. In airplanes with an air data computer, the altimeter will be fed altitude data from the air data computer. The altimeter incorporates a provision to correct for changing air pressure by inputting the altimeter setting obtained from air traffic control or a weather station.
Air Data Computer (ADC)
A device found in transport aircraft and some corporate and smaller aircraft to provide a more accurate readout of pitot/static information (as technology improves and comes down in price, it will be more and more common to see ADCs in light aircraft). Generally, the air data computer will feed information to the altimeters, airspeed indicators, and vertical speed indicators. One exception to this, is that the standby altimeter and standby airspeed indicator will be connected directly to their pitot source(s) and static sources. Air data computers are capable of calculating several additional pieces of information not available from traditional pitot/static systems.
Airspeed
There are three airspeeds a pilot is principally concerned with. Those are indicated airspeed, true airspeed and groundspeed (two other airspeeds you'll see from time-to-time are calibrated airspeed and equivalent airspeed (those are briefly discussed below).
Indicated Airspeed (IAS)
This is the speed to which you will most commonly refer. It is the speed displayed on the airspeed indicator in the cockpit. When operating MSFS on the
VATSIM network, make sure you have selected indicated airspeed and not true airspeed to be displayed on your airspeed indicator (use the path Aircraft ->
Realism Settings -> Display Indicated Airspeed). This is important, because whenever a controller references a speed, he/she is referring to indicated
airspeed. For example, if a controller instructs you to "maintain 250 knots", he/she wants you to maintain that indicated airspeed. The difference between
indicated and true airspeed can be very large at higher altitudes.
IAS is also the most important airspeed number to a pilot. It is the speed he/she will reference for takeoff and landing and maneuvering flight. Simply, it is the
speed that defines if the airplane flies. If a few factors are held constant, an Airplane in level flight will always stall at a constant indicated speed. Takeoff speeds, landing speeds, and minimum maneuvering speeds are always indicated airspeeds. This is why indicated speed is always the primary reference.
Calibrated Airspeed (CAS) / Equivalent Airspeed (EAS)
These speeds will rarely be referenced by the real world or VATSIM pilot. You may run across them in your studies, so we'll briefly discuss them. Both CAS and EAS are refinements to IAS.
CAS is indicated airspeed corrected for instrument and position error. Position error is the main correction to calculate calibrated airspeed. The pitot tube assembly is oriented so that it is most accurate at lower angles of attack; as the angle of attack increases, the airstream strikes the pitot tube at an increasing angle and causes the airspeed
indicator to show a value less than what it should be. Each airplane comes with a chart which corrects indicated airspeed to calibrated airspeed. Additionally, larger airplanes with air data computers are able to correct for position error and some actually display calibrated airspeed on the airspeed indicator.
EAS compensates for the fact that at higher airspeeds and altitudes the compressibility of the air can cause the airspeed indicator to read erroneously high. Most RW pilots don't really understand or need to understand EAS, the same goes for VATSIM pilots (if you'd like to learn more about it, there are plenty of good aerodynamics textbooks - also a search of the web will yield several results).
Like was said at the beginning, CAS and EAS aren't terribly important concepts to a pilot. When you see them, think of them as a refined and more accurate IAS.
True Airspeed (TAS)
TAS is an important flight planning number. With no wind, TAS will be your groundspeed and therefore will dictate how long your flight will take. It
is also the speed which you will file in your flight plan. TAS is IAS (technically it's EAS, but for simplicity we?ll call it IAS) corrected for
altitude and temperature; as the temperature or altitude increases, the air density will decrease and this will cause the indicated airspeed to read lower
than the true airspeed. At sea level on a 15°C day, IAS will be the same as TAS. As altitude increases, the difference between TAS and IAS will increase. At
10,000' at -5°C, 250 knots IAS will give you about 290 knots TAS; at 20,000' at -25°C, 250 knot IAS will give you 335 knots TAS; at 30,000' and -45°C, 250 knots
IAS will give you about a 395 knot TAS. A good rule of thumb to approximate the difference between IAS and TAS is a 2% difference per 1000' increase in
altitude. To calculate a more accurate TAS, use this link∞ to find a TAS calculator (it will calculate other airspeeds as well). If you are flying an
aircraft with an air data computer, there should be a display of TAS somewhere in the cockpit. Again, to reiterate what's been said before, the airspeed
indicator will not show true airspeed. When filing your flight plan, make sure you use TAS in the airspeed block; air traffic controllers will sometimes use
this speed to decide how best to handle your flight. The difference between TAS and IAS is quite large at high altitudes.
Ground Speed (GS)
Ground speed is TAS corrected for tailwind or headwind component. Most of the time, the wind will not be a direct headwind or tailwind. You must figure
out what portion of the wind is acting as a tailwind/headwind and then use that number to correct TAS and come up with a groundspeed. In aircraft with an FMS, GPS, or other area navigation system, there should be a display of groundspeed somewhere in the cockpit.
Mach number
Mach number is a speed derived in reference to the speed of sound. Mach 1 equals the speed of sound; most transport jets (I guess all now that the
Concorde has been removed from service), cannot cruise at Mach 1, but can cruise at a certain ratio of the speed of sound. That ratio is known as the "mach
number". Mach .80 is a speed that is 80% of the speed of sound at the temperature existing outside the aircraft. The speed of sound is a function of
temperature only (not altitude); so at 35000' with an OAT of -50°C, mach .80 will give a TAS of 465 knots, if the temperature was -30°C at 35000', mach .80
would give a TAS of 485 knots. Again, note that altitude is not a factor in the calculation, only air temperature (use this link∞ to calculate the speed of
sound at various temperatures). At higher altitudes (generally beginning around FL280), most jets will fly a particular mach number instead of indicated
airspeed. There are two limiting speeds for aircraft capable of high altitude flight. One is an indicated airspeed; the other is a mach number. The mach
number generally becomes more restrictive at altitudes above the mid-twenties.
At higher altitudes, pilots plan cruise speeds using mach numbers instead of using an indicated speed. At higher altitudes, controllers may assign
speeds expressed as mach numbers instead of indicated airspeed. The phraseology should be something like, "maintain mach point seven four". In that case, the
pilot will maintain a true airspeed that is 74% of the speed of sound at the present outside air temperature. To determine mach number, most airplanes
capable of high altitude flight use an air data computer which calculates mach number and displays it somewhere in the cockpit (commonly using the notation
"M.74" for mach point seven four). If you plan a high altitude flight, know where this information is displayed in your airplane.
Altitudes
When a controller assigns an altitude he/she assigns it with reference to sea level. So when you're given a clearance to "maintain eight thousand", that
means you are required to maintain an altitude 8000' above sea level (a few countries in the world, like Russia and China, assign altitudes in meters -
obviously, that's an important thing to know).
Above ground level (AGL)
Altitude expressed as an altitude above the terrain/obstructions below the aircraft. Obviously this is an important altitude to know. Light aircraft
generally have no direct readout of AGL altitude. Larger aircraft will have a radio altimeter (sometimes called a "radar altimeter") which constantly measures
the airplanes altitude above the ground directly below.
Because the atmospheric air pressure varies, altimeters incorporate a method to compensate for this varying air pressure called the "altimeter setting".
Weather stations measure the air pressure and calculate altimeter settings which are available to pilots. This setting tells the altimeter where sea level would
be. Then the altimeter determines the actual atmospheric pressure outside the airplane and converts it to the actual altitude. Altimeter settings are expressed in inches of mercury (" Hg) or millibars (mb) [1 millibar = 1 hectopascal (hPa)]. Obviously, it's important to know which one the country you'll fly in uses. If your airplane's altimeter does not use the same unit of measure as the altimeter setting provided, you can convert between millibars and inches of mercury by using a factor of 33.86 (1"Hg = 33.86 mb...approximately anyway). To convert from inches of mercury to millibars, multiply by 33.86; to convert from millibars to inches of mercury, divide by 33.86.
Transition altitude / level
To simplify ATC, aircraft flying above a certain altitude will always use a constant altimeter setting of 29.92" Hg/1013 mb. This altitude is called the "transition altitude", and it varies from country to country. In the U.S., it's 18000', in Germany it's 5000', in Japan it's 14000', in Australia it's 10000'; obviously, check on the actual number for the country in which you'll operate. During descent, the altimeter setting is changed from standard (29.92"Hg or 1013 mb) at the "transition level". Just like the transition altitude, this varies from country to country. To make things more complicated, the transition altitude and level aren't always the same. In some cases (like Germany), the transition level can vary from day-to-day and is passed to the pilot by the air traffic controller. Obviously, the specifics of the transition altitude and level vary greatly by region and country; consult your local VATSIM resources for more information.
High altitude flight
At high altitudes and weights, airplanes often are not able to initially climb to their final cruise altitude. For a given weight, each aircraft has a minimum speed and maximum speed at a particular altitude. The theory behind these speeds is complex and beyond the scope of this lesson. There needs to be a safe margin between the lowest speed and highest speed at the cruise altitude. At very high gross weights and very high altitudes, it's possible that the minimum and maximum speeds can be very close (a condition commonly known as "coffins corner" because of the precarious situation it places the airplane). To avoid unsafe cruising conditions, most airplanes have a chart which dictates the maximum altitude for a given weight. Because higher altitudes normally yield better fuel consumption, long range flights often will "step-climb" as they get
lighter (because of fuel burn). If you plan to step-climb during your flight, file your initial cruise altitude request in your flight plan and when you're ready for a further climb, advise the controller of your new request.
There are several factors for a pilot to consider when deciding what altitude to use for cruise flight. In this article we will discuss several of these factors and how they affect both VFR and IFR flights. This is meant to be a basic survey of altitude planning. Some of the subjects discussed are complex; other articles cover some of these subjects in greater detail.
VFR considerations
The first concern for any pilot in choosing a cruise altitude is avoiding terrain and obstacles along the route. Most navigation charts include at least some terrain and obstruction information. It's usually a good idea to fly at least 1000' above the terrain along your route; however, it is sometimes permissible to fly closer to the ground and in some cases you be required to fly higher than 1000' AGL. The regulations on this subject vary based on what you are flying over (generally, you must fly at a higher altitude over congested areas than you would over unpopulated areas) and the country you're flying in. Also, each country establishes a maximum altitude at which VFR flight may be conducted.
This maximum altitude varies considerably across the world; in the U.S., VFR flight may not be conducted at or above 18,000' MSL.
VFR flights also must remain clear of clouds (in some cases a minimum distance from clouds is prescribed) and maintain a certain visibility. These weather minimums for VFR flight vary based on several factors and also vary between countries.
IFR considerations
Just like a VFR pilot, IFR pilots must consider the terrain along their route of flight. Since they may not be able to see the terrain, regulations specify a minimum height above terrain for IFR flights. In this U.S., this minimum altitude is 1000' AGL; in mountainous areas (in the U.S., this is defined by FAR 95), pilots must fly at least 2000' AGL.
If your IFR route will be on airways, those airways will provide the minimum enroute altitude (MEA) for each segment. The MEA assures terrain clearance and also the ability to receive the ground-based navigation aids which define the centerline of the airway. If you plan to operate along an airway, SID or STAR, review the charted minimum enroute altitudes and choose a cruise altitude that is at/above the MEA.
There is also another altitude sometimes published known as the minimum obstruction clearance altitude (MOCA). When published, it applies within 22 NM of the
navigation aid defining the airway centerline; at points more than 22 NM from the navigation aid defining the route, the MEA applies.
With the advent of GPS and other forms of area navigation, it's very common to conduct flights directly between fixes (other articles cover route selection and when direct
routes are appropriate). These direct routes will not have an MEA like an airway. In this case it is up to the pilot to determine an appropriate altitude. Some low altitude IFR charts give a "Grid MORA" (MORA stands for Minimum Off Route Altitude). Those MORAs are very helpful and if you fly at or above the MORA you will always maintain a safe altitude above terrain and obstructions; however, it might be permissible to fly at a lower altitude. The best source for additional terrain information (in the U.S. at least) is sectional charts. Sectionals are generally thought of as VFR charts; however, they provide the best terrain detail of any aviation chart available to a pilot.
Other articles will cover charting and how MEAs, MOCAs, and MORAs are depicted.
Controllers also have minimum IFR altitudes available to them that they must use for flights not operating on published routes (airway, SID, STAR, etc); in some cases, the controller's minimum altitude might be higher than the grid MORA. These altitudes are not directly available to pilots, but the controller will advise you if his or her minimum IFR altitude is higher than your planned altitude.
Fuel considerations
On longer flights, most pilots will choose cruise altitudes near the service ceiling of their aircraft. Flying higher will almost always result in a decreased fuel burn. There are times this is not true; the first is with a much stronger headwind at the higher altitude. This is intentionally vague, since it will vary based on several factors; an additional ten knots of headwind at an altitude 4000' higher would likely not be enough to offset the higher fuel flow at the lower altitude; an additional 60 knots of headwind likely would be enough difference to make the lower altitude more fuel efficient (this large of a wind difference over 4000' of altitude is very rare). At very high weights in transport jets, the aerodynamic effects at high altitudes can make those higher altitudes less fuel efficient; this lower fuel efficiency is why longer range flights often "step climb" over the course of the flight as they burn off fuel (and the aircraft's weight decreases). For example, a 747 flying from Chicago to Hong Kong might initially level off at 30,000', then every couple of hours climb 2000' until reaching 38,000 or 40,000'. This is a fairly complex topic and further discussion is beyond the scope of this article; the documentation with
your virtual aircraft might provide some additional guidance.
Time considerations
Sometimes a pilot will be more concerned with a quick flight than with fuel burn. In this case you'd generally want to choose the "fastest altitude" for your aircraft. Determining the fastest altitude will take some research; use the documentation provided with your aircraft or just fly and experiment to find what works best. Obviously, headwind and tailwind components can make this calculation more complex, the following assumes that there is no wind.
For jet aircraft, the fastest altitude will usually be the altitude at which the maximum indicated airspeed equals the maximum mach number for your aircraft. For most transport
jet aircraft, this will be somewhere between 26000 and 30000 feet. For turboprop aircraft, this will usually be the maximum altitude at which the maximum cruise torque can be maintained. For turbo/supercharged reciprocating powered aircraft, the maximum speed can usually be attained at the maximum altitude which permits maximum cruise manifold pressure to be maintained. For normally aspirated (non-turbo/supercharged) reciprocating aircraft, consult the documentation with the aircraft to find the maximum true airspeed.
Direction of flight
Each country establishes general rules regarding proper altitudes for direction of flight. In the U.S., eastbound IFR flights (defined as a magnetic course of 0-179 degrees) generally fly at odd thousands of feet (e.g. 7000', 15000', 33000', etc.); whereas, westbound IFR flights generally fly at even thousands of feet. Other areas of the world divide even/odd altitudes so it's split between north and southbound aircraft instead of east and westbound aircraft. Air traffic controllers can approve deviations from these altitudes, but will usually try to get pilots to comply since it makes traffic separation easier. At altitudes above 41000', 2000' of vertical separation is required between aircraft, so the
east/west altitudes alternate (e.g. Westbound flights use FL430, FL470, FL510, etc.; Eastbound flights use FL450, FL490, etc).
In the U.S., VFR flights follow the same rules as IFR flights except they add 500' to their altitude (e.g. Westbound VFR flights could cruise at 8500', an eastbound VFR flight might cruise at 5500'). An exception to this is for operations at/below 3000' of the surface; in these cases, VFR aircraft may operate at any altitude consistent with safe flight.
Oxygen / Pressurization requirements
For flight above a certain altitude, unpressurized flight requires the use of supplemental oxygen. While it's not necessary to use oxygen in front of your computer, if you choose to simulate all details of real world operations, you should consider the oxygen rules when choosing a cruise altitude. Each country will have its own rules; in the U.S.,
all unpressurized flights conducted above 14000' require the use of oxygen (it is required at lower altitudes for some flights beyond a certain amount of time). On an unpressurized flight conducted for hire (air taxi or air carrier), the pilots must use oxygen for flights at altitudes above 10000' (oxygen must be available for passengers at higher altitudes depending on several factors).
Flying an airplane requires reference to several different airspeeds and altitudes. There are several important concepts to understand to successful operate your airplane
and to interact with air traffic controllers.
Abbreviations / Definitions
ATC: Air Traffic Control/Controller
Nautical mile (NM): 1 NM = 1.85 km = 1.15 statue miles, approximately anyway; more precisely, a nautical mile is defined as one minute of latitude - there are 60 minutes in one degree, so 1 degree of latitude equals exactly 60 nautical miles.
Knot: A measure of speed representing one nautical mile per hour.
IAS: Indicated Air Speed
TAS: True Air Speed
GS: Ground Speed
MSL: Mean Sea Level - altitude expressed in distance above Mean Sea Level
ASL: same as MSL, except the "A" stands for "above"
AGL: Above Ground Level - altitude expressed in distance above the ground
How Do We Measure and display Airspeed and Altitude?
Pitot Tube / System
The pitot tube is a device mounted outside the airplane to measure airspeed. It is aligned with the airstream so that it can measure the pressure of the air created by the air moving over the airplane's wings. In light aircraft the pitot tube is connected directly to the airspeed indicator; larger aircraft generally incorporate an air data computer and in
those cases the pitot tube is connected to that computer. Light aircraft will normally have one pitot tube, transport aircraft will have at least three.
Static Port / System
Static port(s) are mounted outside the aircraft to measure static air pressure. It is oriented so that it is unaffected by the changes in air pressure caused by the movement of the airplane through the air. In light aircraft, the static port(s) are connected directly to the altimeter, airspeed indicator, and vertical speed indicator. In larger aircraft, static ports are connected to the air data computer, which will calculate various pieces of information based on the static input. Small aircraft generally have one or two static ports (the second port allows the system to compensate for times when the airplane isn't in perfectly coordinated flight). Transport aircraft will normally have at least six static sources.
Airspeed indicator
The airspeed indicator compares pitot air pressure to static air pressure and displays the result as an "indicated airspeed". In airplanes with an air data computer, the airspeed indicator itself is fed airspeed data from the air data computer.
Altimeter
An instrument which measures outside air pressure from the static system and converts that pressure to an altitude. In airplanes with an air data computer, the altimeter will be fed altitude data from the air data computer. The altimeter incorporates a provision to correct for changing air pressure by inputting the altimeter setting obtained from air traffic control or a weather station.
Air Data Computer (ADC)
A device found in transport aircraft and some corporate and smaller aircraft to provide a more accurate readout of pitot/static information (as technology improves and comes down in price, it will be more and more common to see ADCs in light aircraft). Generally, the air data computer will feed information to the altimeters, airspeed indicators, and vertical speed indicators. One exception to this, is that the standby altimeter and standby airspeed indicator will be connected directly to their pitot source(s) and static sources. Air data computers are capable of calculating several additional pieces of information not available from traditional pitot/static systems.
Airspeed
There are three airspeeds a pilot is principally concerned with. Those are indicated airspeed, true airspeed and groundspeed (two other airspeeds you'll see from time-to-time are calibrated airspeed and equivalent airspeed (those are briefly discussed below).
Indicated Airspeed (IAS)
This is the speed to which you will most commonly refer. It is the speed displayed on the airspeed indicator in the cockpit. When operating MSFS on the
VATSIM network, make sure you have selected indicated airspeed and not true airspeed to be displayed on your airspeed indicator (use the path Aircraft ->
Realism Settings -> Display Indicated Airspeed). This is important, because whenever a controller references a speed, he/she is referring to indicated
airspeed. For example, if a controller instructs you to "maintain 250 knots", he/she wants you to maintain that indicated airspeed. The difference between
indicated and true airspeed can be very large at higher altitudes.
IAS is also the most important airspeed number to a pilot. It is the speed he/she will reference for takeoff and landing and maneuvering flight. Simply, it is the
speed that defines if the airplane flies. If a few factors are held constant, an Airplane in level flight will always stall at a constant indicated speed. Takeoff speeds, landing speeds, and minimum maneuvering speeds are always indicated airspeeds. This is why indicated speed is always the primary reference.
Calibrated Airspeed (CAS) / Equivalent Airspeed (EAS)
These speeds will rarely be referenced by the real world or VATSIM pilot. You may run across them in your studies, so we'll briefly discuss them. Both CAS and EAS are refinements to IAS.
CAS is indicated airspeed corrected for instrument and position error. Position error is the main correction to calculate calibrated airspeed. The pitot tube assembly is oriented so that it is most accurate at lower angles of attack; as the angle of attack increases, the airstream strikes the pitot tube at an increasing angle and causes the airspeed
indicator to show a value less than what it should be. Each airplane comes with a chart which corrects indicated airspeed to calibrated airspeed. Additionally, larger airplanes with air data computers are able to correct for position error and some actually display calibrated airspeed on the airspeed indicator.
EAS compensates for the fact that at higher airspeeds and altitudes the compressibility of the air can cause the airspeed indicator to read erroneously high. Most RW pilots don't really understand or need to understand EAS, the same goes for VATSIM pilots (if you'd like to learn more about it, there are plenty of good aerodynamics textbooks - also a search of the web will yield several results).
Like was said at the beginning, CAS and EAS aren't terribly important concepts to a pilot. When you see them, think of them as a refined and more accurate IAS.
True Airspeed (TAS)
TAS is an important flight planning number. With no wind, TAS will be your groundspeed and therefore will dictate how long your flight will take. It
is also the speed which you will file in your flight plan. TAS is IAS (technically it's EAS, but for simplicity we?ll call it IAS) corrected for
altitude and temperature; as the temperature or altitude increases, the air density will decrease and this will cause the indicated airspeed to read lower
than the true airspeed. At sea level on a 15°C day, IAS will be the same as TAS. As altitude increases, the difference between TAS and IAS will increase. At
10,000' at -5°C, 250 knots IAS will give you about 290 knots TAS; at 20,000' at -25°C, 250 knot IAS will give you 335 knots TAS; at 30,000' and -45°C, 250 knots
IAS will give you about a 395 knot TAS. A good rule of thumb to approximate the difference between IAS and TAS is a 2% difference per 1000' increase in
altitude. To calculate a more accurate TAS, use this link∞ to find a TAS calculator (it will calculate other airspeeds as well). If you are flying an
aircraft with an air data computer, there should be a display of TAS somewhere in the cockpit. Again, to reiterate what's been said before, the airspeed
indicator will not show true airspeed. When filing your flight plan, make sure you use TAS in the airspeed block; air traffic controllers will sometimes use
this speed to decide how best to handle your flight. The difference between TAS and IAS is quite large at high altitudes.
Ground Speed (GS)
Ground speed is TAS corrected for tailwind or headwind component. Most of the time, the wind will not be a direct headwind or tailwind. You must figure
out what portion of the wind is acting as a tailwind/headwind and then use that number to correct TAS and come up with a groundspeed. In aircraft with an FMS, GPS, or other area navigation system, there should be a display of groundspeed somewhere in the cockpit.
Mach number
Mach number is a speed derived in reference to the speed of sound. Mach 1 equals the speed of sound; most transport jets (I guess all now that the
Concorde has been removed from service), cannot cruise at Mach 1, but can cruise at a certain ratio of the speed of sound. That ratio is known as the "mach
number". Mach .80 is a speed that is 80% of the speed of sound at the temperature existing outside the aircraft. The speed of sound is a function of
temperature only (not altitude); so at 35000' with an OAT of -50°C, mach .80 will give a TAS of 465 knots, if the temperature was -30°C at 35000', mach .80
would give a TAS of 485 knots. Again, note that altitude is not a factor in the calculation, only air temperature (use this link∞ to calculate the speed of
sound at various temperatures). At higher altitudes (generally beginning around FL280), most jets will fly a particular mach number instead of indicated
airspeed. There are two limiting speeds for aircraft capable of high altitude flight. One is an indicated airspeed; the other is a mach number. The mach
number generally becomes more restrictive at altitudes above the mid-twenties.
At higher altitudes, pilots plan cruise speeds using mach numbers instead of using an indicated speed. At higher altitudes, controllers may assign
speeds expressed as mach numbers instead of indicated airspeed. The phraseology should be something like, "maintain mach point seven four". In that case, the
pilot will maintain a true airspeed that is 74% of the speed of sound at the present outside air temperature. To determine mach number, most airplanes
capable of high altitude flight use an air data computer which calculates mach number and displays it somewhere in the cockpit (commonly using the notation
"M.74" for mach point seven four). If you plan a high altitude flight, know where this information is displayed in your airplane.
Altitudes
When a controller assigns an altitude he/she assigns it with reference to sea level. So when you're given a clearance to "maintain eight thousand", that
means you are required to maintain an altitude 8000' above sea level (a few countries in the world, like Russia and China, assign altitudes in meters -
obviously, that's an important thing to know).
Above ground level (AGL)
Altitude expressed as an altitude above the terrain/obstructions below the aircraft. Obviously this is an important altitude to know. Light aircraft
generally have no direct readout of AGL altitude. Larger aircraft will have a radio altimeter (sometimes called a "radar altimeter") which constantly measures
the airplanes altitude above the ground directly below.
Because the atmospheric air pressure varies, altimeters incorporate a method to compensate for this varying air pressure called the "altimeter setting".
Weather stations measure the air pressure and calculate altimeter settings which are available to pilots. This setting tells the altimeter where sea level would
be. Then the altimeter determines the actual atmospheric pressure outside the airplane and converts it to the actual altitude. Altimeter settings are expressed in inches of mercury (" Hg) or millibars (mb) [1 millibar = 1 hectopascal (hPa)]. Obviously, it's important to know which one the country you'll fly in uses. If your airplane's altimeter does not use the same unit of measure as the altimeter setting provided, you can convert between millibars and inches of mercury by using a factor of 33.86 (1"Hg = 33.86 mb...approximately anyway). To convert from inches of mercury to millibars, multiply by 33.86; to convert from millibars to inches of mercury, divide by 33.86.
Transition altitude / level
To simplify ATC, aircraft flying above a certain altitude will always use a constant altimeter setting of 29.92" Hg/1013 mb. This altitude is called the "transition altitude", and it varies from country to country. In the U.S., it's 18000', in Germany it's 5000', in Japan it's 14000', in Australia it's 10000'; obviously, check on the actual number for the country in which you'll operate. During descent, the altimeter setting is changed from standard (29.92"Hg or 1013 mb) at the "transition level". Just like the transition altitude, this varies from country to country. To make things more complicated, the transition altitude and level aren't always the same. In some cases (like Germany), the transition level can vary from day-to-day and is passed to the pilot by the air traffic controller. Obviously, the specifics of the transition altitude and level vary greatly by region and country; consult your local VATSIM resources for more information.
High altitude flight
At high altitudes and weights, airplanes often are not able to initially climb to their final cruise altitude. For a given weight, each aircraft has a minimum speed and maximum speed at a particular altitude. The theory behind these speeds is complex and beyond the scope of this lesson. There needs to be a safe margin between the lowest speed and highest speed at the cruise altitude. At very high gross weights and very high altitudes, it's possible that the minimum and maximum speeds can be very close (a condition commonly known as "coffins corner" because of the precarious situation it places the airplane). To avoid unsafe cruising conditions, most airplanes have a chart which dictates the maximum altitude for a given weight. Because higher altitudes normally yield better fuel consumption, long range flights often will "step-climb" as they get
lighter (because of fuel burn). If you plan to step-climb during your flight, file your initial cruise altitude request in your flight plan and when you're ready for a further climb, advise the controller of your new request.
There are several factors for a pilot to consider when deciding what altitude to use for cruise flight. In this article we will discuss several of these factors and how they affect both VFR and IFR flights. This is meant to be a basic survey of altitude planning. Some of the subjects discussed are complex; other articles cover some of these subjects in greater detail.
VFR considerations
The first concern for any pilot in choosing a cruise altitude is avoiding terrain and obstacles along the route. Most navigation charts include at least some terrain and obstruction information. It's usually a good idea to fly at least 1000' above the terrain along your route; however, it is sometimes permissible to fly closer to the ground and in some cases you be required to fly higher than 1000' AGL. The regulations on this subject vary based on what you are flying over (generally, you must fly at a higher altitude over congested areas than you would over unpopulated areas) and the country you're flying in. Also, each country establishes a maximum altitude at which VFR flight may be conducted.
This maximum altitude varies considerably across the world; in the U.S., VFR flight may not be conducted at or above 18,000' MSL.
VFR flights also must remain clear of clouds (in some cases a minimum distance from clouds is prescribed) and maintain a certain visibility. These weather minimums for VFR flight vary based on several factors and also vary between countries.
IFR considerations
Just like a VFR pilot, IFR pilots must consider the terrain along their route of flight. Since they may not be able to see the terrain, regulations specify a minimum height above terrain for IFR flights. In this U.S., this minimum altitude is 1000' AGL; in mountainous areas (in the U.S., this is defined by FAR 95), pilots must fly at least 2000' AGL.
If your IFR route will be on airways, those airways will provide the minimum enroute altitude (MEA) for each segment. The MEA assures terrain clearance and also the ability to receive the ground-based navigation aids which define the centerline of the airway. If you plan to operate along an airway, SID or STAR, review the charted minimum enroute altitudes and choose a cruise altitude that is at/above the MEA.
There is also another altitude sometimes published known as the minimum obstruction clearance altitude (MOCA). When published, it applies within 22 NM of the
navigation aid defining the airway centerline; at points more than 22 NM from the navigation aid defining the route, the MEA applies.
With the advent of GPS and other forms of area navigation, it's very common to conduct flights directly between fixes (other articles cover route selection and when direct
routes are appropriate). These direct routes will not have an MEA like an airway. In this case it is up to the pilot to determine an appropriate altitude. Some low altitude IFR charts give a "Grid MORA" (MORA stands for Minimum Off Route Altitude). Those MORAs are very helpful and if you fly at or above the MORA you will always maintain a safe altitude above terrain and obstructions; however, it might be permissible to fly at a lower altitude. The best source for additional terrain information (in the U.S. at least) is sectional charts. Sectionals are generally thought of as VFR charts; however, they provide the best terrain detail of any aviation chart available to a pilot.
Other articles will cover charting and how MEAs, MOCAs, and MORAs are depicted.
Controllers also have minimum IFR altitudes available to them that they must use for flights not operating on published routes (airway, SID, STAR, etc); in some cases, the controller's minimum altitude might be higher than the grid MORA. These altitudes are not directly available to pilots, but the controller will advise you if his or her minimum IFR altitude is higher than your planned altitude.
Fuel considerations
On longer flights, most pilots will choose cruise altitudes near the service ceiling of their aircraft. Flying higher will almost always result in a decreased fuel burn. There are times this is not true; the first is with a much stronger headwind at the higher altitude. This is intentionally vague, since it will vary based on several factors; an additional ten knots of headwind at an altitude 4000' higher would likely not be enough to offset the higher fuel flow at the lower altitude; an additional 60 knots of headwind likely would be enough difference to make the lower altitude more fuel efficient (this large of a wind difference over 4000' of altitude is very rare). At very high weights in transport jets, the aerodynamic effects at high altitudes can make those higher altitudes less fuel efficient; this lower fuel efficiency is why longer range flights often "step climb" over the course of the flight as they burn off fuel (and the aircraft's weight decreases). For example, a 747 flying from Chicago to Hong Kong might initially level off at 30,000', then every couple of hours climb 2000' until reaching 38,000 or 40,000'. This is a fairly complex topic and further discussion is beyond the scope of this article; the documentation with
your virtual aircraft might provide some additional guidance.
Time considerations
Sometimes a pilot will be more concerned with a quick flight than with fuel burn. In this case you'd generally want to choose the "fastest altitude" for your aircraft. Determining the fastest altitude will take some research; use the documentation provided with your aircraft or just fly and experiment to find what works best. Obviously, headwind and tailwind components can make this calculation more complex, the following assumes that there is no wind.
For jet aircraft, the fastest altitude will usually be the altitude at which the maximum indicated airspeed equals the maximum mach number for your aircraft. For most transport
jet aircraft, this will be somewhere between 26000 and 30000 feet. For turboprop aircraft, this will usually be the maximum altitude at which the maximum cruise torque can be maintained. For turbo/supercharged reciprocating powered aircraft, the maximum speed can usually be attained at the maximum altitude which permits maximum cruise manifold pressure to be maintained. For normally aspirated (non-turbo/supercharged) reciprocating aircraft, consult the documentation with the aircraft to find the maximum true airspeed.
Direction of flight
Each country establishes general rules regarding proper altitudes for direction of flight. In the U.S., eastbound IFR flights (defined as a magnetic course of 0-179 degrees) generally fly at odd thousands of feet (e.g. 7000', 15000', 33000', etc.); whereas, westbound IFR flights generally fly at even thousands of feet. Other areas of the world divide even/odd altitudes so it's split between north and southbound aircraft instead of east and westbound aircraft. Air traffic controllers can approve deviations from these altitudes, but will usually try to get pilots to comply since it makes traffic separation easier. At altitudes above 41000', 2000' of vertical separation is required between aircraft, so the
east/west altitudes alternate (e.g. Westbound flights use FL430, FL470, FL510, etc.; Eastbound flights use FL450, FL490, etc).
In the U.S., VFR flights follow the same rules as IFR flights except they add 500' to their altitude (e.g. Westbound VFR flights could cruise at 8500', an eastbound VFR flight might cruise at 5500'). An exception to this is for operations at/below 3000' of the surface; in these cases, VFR aircraft may operate at any altitude consistent with safe flight.
Oxygen / Pressurization requirements
For flight above a certain altitude, unpressurized flight requires the use of supplemental oxygen. While it's not necessary to use oxygen in front of your computer, if you choose to simulate all details of real world operations, you should consider the oxygen rules when choosing a cruise altitude. Each country will have its own rules; in the U.S.,
all unpressurized flights conducted above 14000' require the use of oxygen (it is required at lower altitudes for some flights beyond a certain amount of time). On an unpressurized flight conducted for hire (air taxi or air carrier), the pilots must use oxygen for flights at altitudes above 10000' (oxygen must be available for passengers at higher altitudes depending on several factors).