Flight Instruments: Altimeter

Your height above the terrain and in relation to other aircraft is critically important. The altimeter is an amazing instrument that makes it easy to tell what your altitude is.

How to read an altimeter

There are 3 needles on this standard altimeter. The long needles pointing near the 7 moves the fastest and indicates hundreds of feet. The short needles pointing near the 1 indicates thousands and the thin needle pointing near 0 indicates tens of thousands. Continue reading “Flight Instruments: Altimeter”

Flight Instruments: Airspeed Indicator

The speed of an aircraft through the air determines its performance in many ways.

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How does the indicator work?

A basic airspeed indicator is a mechanical device that compares pressure from the pitot tube to pressure from the static port. The static port is mounted sideways with a hole the doesn’t face directly into the oncoming air. This way it gets a “static” measure of the air pressure. The pitot tube has a hole that does face into the airstream, so it has oncoming air forced directly into it.

The air from the pitot tube fills a diaphragm (like an accordion) and makes it expand. The air from the static port fills the gauge around the diaphragm and pushes it to contract. As the diaphragm expands and contracts it pushes a needle that we see on the instrument.

Airspeed Indicator (Source: Wikipedia)

The airspeed indicator is very reliable but there are a few things that can go wrong.

Reading the airspeed indicator

The airspeed indicator is fairly self-explanatory to read. The most common mistake is not paying attention to units. Sometimes the instrument will measure miles per hour instead of knots. Always make sure you know which one you are looking at.

The indicator can also have some error, especially at high angles of attack. The manufacturer will often publish a calibrated airspeed table to help you determine the difference.

6 Types of Airspeed

1. Indicated: IAS

Indicated airspeed is the speed that the plane “feels”. It might help to think of it as the number of air molecules hitting the plane. This is the speed that matters for the performance of the plane. It can be read directly on the airspeed indicator.

2. True: TAS

As you climb the air gets thinner. As the air gets thinner there are fewer air molecules in a given volume of air. This allows the aircraft to fly faster.

For example, if your plane has enough power to fly at 100 knots and you maintain 100 knots while climbing your true airspeed will increase. True airspeed is your actual speed through the mass of air. As you climb and the air thins out, if you are still at 100 knots then you are still encountering the same amount of air over time, but since that air is spread out over a longer distance you are flying at a faster speed. True airspeed is the same as groundspeed if there is absolutely no wind.

This bonus in speed and better fuel economy are the reasons that planes bother to climb all the way up to high altitudes.

3. Calibrated: CAS

Airspeed indicators aren’t perfect. When flaps are down or the plane is at a high angle of attack the airspeed indicator may be off by several knots. This error is studied and a placard is provided with the correct numbers. So calibrated airspeed is more precise than indicated airspeed but it is not displayed directly on the airspeed indicator.

4. Ground Speed: GS

This is not an airspeed, but it is worth including here. Ground speed is the speed that really matters for getting somewhere, it is very simply your speed over the ground. It is equal to your true airspeed plus or minus a tailwind or headwind.

When there is enough wind it is possible to gain an enormous amount of extra speed. This is why jets love to take advantage of the jet stream where the wind speed can often exceed 100 knots.

It is also possible to make a plane stop or fly backward. See the video below that illustrates this concept.

5. Equivalent Airspeed: EAS

Equivalent airspeed is a further correction of calibrated airspeed that corrects for airspeed indicator errors due to compressibility. It is most prominent at high altitudes and high speeds. Modern planes that can reach these altitudes and speeds generally have an air data computer that handles the calculation of EAS but a simple performance chart can be used as well. For light aircraft, EAS is generally ignored because it is very close to being the same as CAS.

6. Mach Number: M

Aircraft that fly at higher altitudes and speeds, like jets, generally refer to their speed in terms of mach number. This speed is measured as the ratio of the speed of sound. For example, mach 1 means you are flying at the speed of sound, and mach .5 means you are flying at half the speed of sound.

Pilots generally pronounce mach numbers like “mach point seven five”, or “mach point eight” for M.75 and M.8, respectively.

Most planes fly at subsonic speeds, less than the speed of sound.

Faster planes like the Concorde, and some military fighters and bombers can fly at transonic speeds, at the speed of sound. They can then accelerate to supersonic speeds great than the speed of sound.

Anything greater than M5.0 is considered to be a hypersonic speed. Hypersonic aircraft are certainly being studied but as far as I know, there aren’t any flying.

As an aircraft reaches higher altitudes the mach number is used to measure speed instead of IAS. An aircraft is limited in IAS by aerodynamic pressure and in mach number it is limited by aerodynamic shock waves. Since there are two different limits they both need to be considered.

For example, a Boeing 757 has a Vmo (maximum operating airspeed) of 350 knots and an Mmo (maximum mach number) of M0.86.

At a low altitude near sea level M.86 is 568 knots, well beyond the Vmo limit. However, at a high altitude like 40,000 feet M.86 is approximately 262 knots, well below the Vmo limit. This is why pilots will reference IAS at low altitudes and mach number at high altitudes. During climb there is a crossover altitude at which the transition is made from thinking in IAS to thinking in mach.