Flight Instruments

FltInst01This is the instrument panel of a light twin. It may look incomprehensible now, but by the end of this section you will be able to find and recognize any instrument in this or any other panel, and to know what the instrument shows and how it works.

FltInst02Even J-3 Cubs and Aeronca Champs with no electrical system have a pitot-static system. This airplane looks like a SIAI Marchetti SF.260, which I had the pleasure of flying on one memorable flight in 1972. Great performer and aerobatic, too.

FltInst03The pitot-static system looks like this. There is a pitot tube that sticks out into the airstream and senses dynamic pressure. And a static port that senses the pressure outside the aircraft. What is this all for? Well, as you can see in the slide, the airspeed indicator measures the difference between static and dynamic pressure and displays the result as airspeed. This is called IAS, or Indicated air Speed. The other two instruments, the Altimeter and the Vertical Speed Indicator, can derive their indications solely from the static pressure.

FltInst04This is what the pitot tube and the static port look like on the Cessna 172.

FltInst05And this is what you’ll see on Piper aircraft. The hole on the leading edge of this fin is the pitot, the one on the bottom the static port.

FltInst06This is what an airspeed indicator looks like. Noticed the colored bands. The green (a bit hard to see on this slide, but it extends from 65 to 165) is the normal operating range. The low end (65) is the clean stall speed, Vs. The white arc is the flap operating range, from the flaps down stall speed, Vso, (60 ), up to the maximum flap speed Vfe , (for flaps extended – (100)). The yellow arc is the caution range, in this case from 165, normal operating speed, or Vno, to the very important Never Exceed Speed, or Vne.

The curious are directed to the Wikipedia V-speeds articles.

FltInst07How does an airspeed indicator work? You can see there is a diaphragm inside the case. Pitot pressure goes to one side, and static pressure to the other side of the diaphragm. A difference in pressure bends the diaphragm, which then moves the needle through a linkage and sector gear.

Notice that the static system pictured here has two ports linked by a  Y tube. The ports are on opposite sides of the fuselage, cancelling out errors caused by slip angles (the fuselage not meeting the air head-on).

FltInst08What is true airspeed?

As an airplane climbs and the air pressure falls, it has to fly faster through the less-dense air to maintain the same IAS, or dynamic pressure. That, in an important way, is an advantage. Jets fly high, where the air is rare, because they are traveling through the air at nearly twice the speed on the airspeed indicator. They are also getting about twice the miles per gallon they would a low altitude.

This slide is a range diagram for a C-172. The vertical lines are power settings – 75%, 65%, etc. You can see the same gain of true airspeed at higher altitudes we spoke of for jets – just not as much. (KTAS means Knots True Air speed.)

FltInst09This is the big picture, for jets. (The C-172 is still represented, but at the bottom left corner of the diagram, with speeds under 100 KIAS (Knots Indicated Air Speed) and altitudes below 10,000 feet.

If you follow the red line (300 KIAS), you can see that at FL32.5 (32,500 feet) the True Airspeed is 500 Knots and the Mach Number is 0.85. (Mach 1.0 is the speed of sound, which decreases with altitude in the less dense air.) Further study here and here.


True Airspeed (TAS) is different from Indicated Airspeed (IAS) because as we climb, or the air gets warmer, the air is less dense. That means that we have to fly through the air faster to get the same dynamic pressure (IAS). The equation for this relationship is:

{\mathrm  {TAS}}={\mathrm  {EAS}}{\sqrt  {{\frac  {\rho _{0}}{\rho }}}}

Translated, this means that TAS is proportional to the square root of the air density at Sea Level in a standard atmosphere divided  by the air density at the altitude and temperature where you are flying.

OK – so how do we find TAS practically? We use the wheel, the E6-B computer or the Jeppesen equivalent. The are circular slide rules that are (used to be) part of a pilot’s shirt-pocket equipment.

wikitasThis is a part of a page from Wikipedia. The airspeed indicator depicted features a built-in TAS calculator. You can see that if you turn the knob you can align an altitude and a temperature on the top scale (white over black).  Then the TAS appears opposite the IAS under the needle on the bottom white scale.

FltInst10Oh, oh. There’s more. CAS – Calibrated Airspeed. This is what the airspeed indicator would indicate if it were perfect and had no errors. Clear as a bell, right?

The slide shows the relationship between IAS and CAS for the C-172. You can see that around 60 – 70 knots, (approach speed) or even 80 knots, the two values are very close, changing slightly with flap extension. But at very low speeds the airspeed indicator reads low. (Much of the error is the angle of attack of the pitot tube.)

FltInst11And yet more complications! The airplane, once it leaves the ground, is flying through the air. Duh, you say.

But take a moment to do a thought experiment: imagine the moment the airplane leaves the ground. Or better yet, imagine a C-172 taking off from an aircraft carrier. Let’s say the carrier is steaming (nuking?) into the wind at 20 knots. The wind is 40 gusting to fifty knots. There are chocks behind the wheels. Assistants are standing behind the wing struts, steadying the airplane. You are ready. The controls are alive – you could rock the wings, but you don’t. You apply full power. The assistants step away to the side. You are flying before the wheels even move.

We have just used our imagination to analyze a situation where the headwind is equal to the airspeed required for takeoff. Here is a short takeoff and landing contest at Oshkosh, just for fun. Aircraft like these (say a J-3 or a Champ) cruise at about 80-90 mph. So if you;re flying cross-country, following an Interstate highway, and you have a headwind, it’s not unusual to find that the cars are passing you.

Where all this gets really interesting is in takeoffs and landings with crosswinds.

So, yes – once you are airborne you are flying through the air, and your motion over the ground is the sum of two vectors: you through the air, and the wind (which is the air moving relative to the ground).

FltInst12In a way, the altimeter is even simpler than the airspeed indicator. It is connected only to the static pressure line, and reads the pressure in that line. As you climb through the atmosphere, the pressure is less because there is less air above you. The altimeter reads in feet: in the above slide it reads 1300 feet. The big hand is hundreds of feet and the small hand is thousands of feet. Do you see the very small hand? If it were just above the 1, instead of just above the 0, the altimeter would read 11,300 feet. Right: the tiny hand reads tens of thousands of feet.

FltInst13The quick takeaway on altimeter errors: High to Low, Look out Below, and Cold Air, Look out Below.

Why? Since the atmosphere is composed of gasses, it obeys Boyle’s Law and Charles’ Law. In a word, it is compressible. The wrinkle is that these laws describe a gas in a container, while the atmosphere is held to the earth by gravity. It gets bigger (expands further out into space) when it is warmer. The pressure at any point depends on the weight of the column of air above that point. The bottom line is that since the altimeter measures pressure, when you fly into colder air or an area of low pressure, you are lower than your altimeter indicates.

But don’t worry – there is a knob and an altimeter setting scale on the altimeter. You can use it to calibrate your altimeter for the pressure in your area or at your airport, which may be higher or lower than a standard atmosphere, which is 29.92 inches of mercury, or 1013.2 millibars.