Airplane, Altitude, and Airspeed

Airplanes, Altitudes, and Airspeeds

(MSL, ASL, STD pressure, IAS, TAS, high alt flight)

 

By Eric Stearns

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. Click on this link∞ for a ground speed calculator. 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).

Mean Sea Level (MSL) / Above Sea Level (ASL)

Terms describing altitude expressed in height above sea level. All altitudes in ATC are expressed in terms of MSL/ASL; the altimeter on the instrument panel will read altitude above sea level.

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.

Altimeter setting

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.

Altitude versus flight level

This is one area that is commonly misunderstood. Simply, altitudes are in reference to an altimeter setting from a local weather station, and flight levels are in reference to a standard altimeter setting. The world has agreed that a standard altimeter setting is 29.92"Hg or 1013 mb. As was discussed above, the point at which a pilot changes from a local altimeter setting to the standard setting is called the "transition altitude"; the point at which a pilot changes from the standard altimeter setting to the local altimeter setting is called the "transition level". When ATC assigns an altitude below the transition level, it will be expressed in thousands/hundreds of feet/meters. For example, if a pilot is assigned an altitude of 10,000' and that altitude is below the transition altitude, the controller will express it as "one zero thousand". If that same pilot was in a different country and above the transition altitude at 10,000', the controller would express it as "flight level one zero zero".

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.

Flight Planning

There are a few other lessons dedicated to flight planning, but let's take a simple example just to apply what we've learned.

We've decided (again there are other lessons on flight planning, so we're skipping lots of details here) that the following conditions will exist for our cruise portion:

Cruise speed: Mach .75
Altitude: FL300 (we'll assume a standard temperature of -45°C)
Wind: 40 knot tailwind

Questions

1. What will our true airspeed be?

 

We've decided to cruise at mach .75 (which we've learned is 75% of the speed of sound). Using this link∞, we'll first calculate the speed of sound for our temperature of -45°C which is about 590 knots. Remember, that means 590 knots will be Mach 1. We'll cruise at 75% of Mach 1, so multiply 590 by .75 and you get approximately 440 knots. So our true airspeed will be about 440 knots.


2. When we file our flight plan, what airspeed will we enter?

True airspeed, so enter 440 knots


3. At our 440 knot true airspeed, what should we expect to see as an indicated speed?

This isn't a calculation that we'd normally perform for flight planning. However, to reinforce what we've learned, go to the calculators onthis∞ page. Use the one on right titled "calibrated air speed" (remember calibrated air speed is just a slightly refined indicated airspeed) and fill in the blanks with the information from above. You should get about 280 knots. So when you?re level at FL300 and mach .75, you should expect to see around 280 knots on your airspeed indicator.


4. What will our ground speed be?

Ground speed is true airspeed corrected for headwind/tailwind component. We already know our true airspeed is 440 knots, so we add the 40 knot tailwind to it and come up with a 480 knot ground speed. That means we'll move 480 nautical miles for each hour of cruise.


5. While at FL300, the controller asks you to "say airspeed", which type of airspeed should you reply with?

Indicated airspeed - just read it right off your airspeed indicator; a controller might also ask you to "say mach number". I'm sure you know the right answer to that one!