3 Things Every Pilot Needs to Know About… Airspeed Indicators

The airspeed indicator is probably my favorite instrument. I don’t know why and in reality when it comes to flying small planes, if you know what you’re doing, you can fly without one, when required by emergency anyways, since it is a required item for flying under both visual and instrument flight rules.

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While this aircraft (a 1968 Piper Arrow) didn’t have a traditional 6-pack the airspeed indicator is in the familiar location up and left from the pilot side control yoke… You’ll also note this particular airspeed indicator reads airspeed in MPH on the outer scale and Knots on the inside scale… Most (newer) airspeed indicators show Knots on the outer scale.

As part of the “Back to Basics” approach we’re taking here at ReviewBeforeFlight recently, today we’ll explore 3 things you should know about the airspeed indicator in your airplane!

1- What it shows… How fast you’re going!

This one is kind of obvious right? The airspeed indicator does exactly what its name says it should do, provides you with the speed of your airplane. But, there are a lot of different types of airspeed, which one does your airspeed tell you? Indicated airspeed is the speed read but he pilot off the instrument in the aircraft.

Back when I was a student with Delta Connection Academy, they drilled in the I-C-E-T-G acronym for the types of airspeeds and it has stuck with me all these years later.

Indicated – speed read on the face of the instrument

Calibrated – Indicated airspeed adjusted for errors associated with the system (ie- the pitot tube is fixed as the angle of attack of the aircraft changes the air flow into/about the system is slightly different)

Equivalent – Calibrated airspeed adjusted for compression

True – Calibrated or Equivalent airspeed adjusted for non-standard temperature/pressure

Ground (speed) – Speed the aircraft is moving across the ground

2 – How it works… Pressure differences

The airspeed indicator uses a series of mechanical linkages from two separate air (pressure sensor) inputs. One of these sources is a forward facing port, on a pitot tube, which detects/accepts a ram air input. Essentially this is air being forced into the forward-facing port.

The ram air is then compared to a static air input from another port which is side-ways or rear facing. This port is designed to be the baseline that the ram input it compared to. Mechanical linkages convert the differences of these two pressures to the displayed airspeed not he face of the instrument.

This is why an aircraft parked on the ground can display an airspeed if the pitot tube is left uncovered and the prevailing wind is blowing into the open pitot tube port.

3 – Why it’s good to train regularly for not having one!

During initial flight training the simulated loss of the airspeed indicator is a common occurrence during flight training. I know a certain flight instructor who simulates failures of all the standard 6-pack instruments at once to see how the student manages their energy…

Even after earning a private pilot certificate practicing an airspeed indicator failure with a flight instructor every now and again isa good plan. Airspeed indicator failures are rare, but fairly common place. I can recall two examples from my flying, without spending a lot of time thinking about it.

One was an instrument flight training lesson, I was rolling down Runway 14 at New Bedford Regional Airport (KEWB) and has completed my roll calls (engine gauges green, airspeed alive and tach green). Then as our speed was increasing I glanced down and noticed the airspeed was reading zero. I quickly reduced the throttle to idle, added back pressure to slow down, brakes at the proper time and notified the control tower I was aborting takeoff, and would like to taxi back to the “North Ramp”. I also added I did not need assistance. The controller cleared me to taxi back crossing the other runway on my way. Upon shutting the aircraft down on the map, my instructor and I found the remains of a yellow jacket speared by the pitot tube… This was a fairly standard failure. a blockage created a zero reading and we were not he ground the whole time…

Another airspeed indicator failure from my flying was a little less standard. This time I was flying as the instructor. My student had completed a preflight inspection of the airplane as did I. Nothing out of the ordinary was noticed. We took off and everything appeared normal. However on the climb we never got above 60-knots. My student was flying a normal climb out, pitching for 80-knots and to his credit he identified that as he lower the nose more than normal the speed did not increase. We quickly diagnosed that there was something wrong. My student continued in the traffic pattern and flew a solid approach to landing. I didn’t even need to assist… This instance was different because the indicator was apparently operating, it just wasn’t operating correctly. The mechanic found debris consistent with a wasp nest way back int he pitot tube near the back vent, which caused the faulty readings. the blockage couldn’t be seen during the inspection and there was no way for us to know, but my student handled it well nonetheless.

Flying without an airspeed indicator might be out of the ordinary but a landing is all energy management, an airspeed indicator is simply a supporting feature in this, not a primary need, as long as you practice once in a while, it’ll be a non-event when you find yourself without an operable air speed indicator on final for landing.

-Fly Safe, @MTElia1B9

 

 

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