View Full Version : Airspeed Indicator Failure

10th Jan 2008, 03:51
Let's say you have just taken off, and are climbing through 500 ft AGL when suddenly all of the airspeed indicators fail (including backup). What would be the procedure? I'm guessing landing right away, but then how would you know the vital speeds needed for approach?

10th Jan 2008, 04:40
You turn to your "Flight with Unreliable Airspeed" chart. After about 2 or 3 thousand hours in type, your arm should know abut how far to push. Plus you can work back from a GPS or IRU since you know the surface winds.

Old Smokey
10th Jan 2008, 07:17
To follow on from the wise words of MarkerInbound, most modern aircraft have a VLS (V, Low Speed) displayed on the speed tape, or "eyebrows" on the ADI showing maximum pitch angle, all of these are supplied by the Angle of Attack vanes, independant of the manometric system. Staying comfortably above the VLS, and comfortably below the "eyebrows" should keep you safe.:ok:

(Well I lied slightly, they do refer to Static Pressure to compute the stall angle of attack at very high altitudes, but you were referring to the "After Takeoff" case).

After you've landed, buy a lottery ticket, the odds in favour of a dual ADC failure plus a pneumatic standby IAS failure are billions to one against.


Old Smokey

10th Jan 2008, 07:46
Just set standard powers and attitudes for your aircraft to position for an approach. Following prelanding checks set the aircraft up in final approach configuration at a safe altitude and conduct a standard final descent profile, allow the aircraft to stabilise and then trim it out.
Dont touch the trim after that and then position on finals for the runway in use.
During the approach maintain a normal final gradient to the runway. If you are pushing forward then you are fast and if having to pull back then you are slow. This will give you a fairly good feedback of speed.

10th Jan 2008, 08:40
It is a common simulator exercise.

10th Jan 2008, 08:42
Pitch and power always works well, most pilots know what the aircraft speed is for a given configuration and power setting.

10th Jan 2008, 11:35
A long time ago, I flew the Lear 23. What a great airplane! And, it was equipped with a well-calibrated A-O-A indicator. Now, there were later modifications by various outfits who did wing mods (among other things), and I flew the 23 with Century mod, the Raisbeck mod, etc., and those outfits put in an even more refined A-O-A system.

Anyway, it got to the point where you really didn't need any airspeed indicator at all. Really, just pitch, thrust, and A-O-A. It worked well for working out flaps on approach, retracting flaps after takeoff, long range cruise, high speed cruise...you name it, the A-O-A indicator worked well. And, now, later, that we're doing the windshear recovery drills...it's a great tool for that, too.

I really don't understand why we do have these neat things in 'big airplanes'....I mean well-calibrated indicators. Maybe Old Smokey can answer this for me.

But, I remember I got to the point where I used the A-O-A indicator for best rate of climb, best angle of climb....if you know what angle is correct for what you want to do...you're spot on target. (And, use the airpseed/MACH indicator as a supporting instrument.)

I guess, to a degree, we use the FPV in a similar manner...but, you have to go through all this mess in the abnormal/emergency checklist...You go to the column that has your approximate weight...then, you cross reference with configuration...get your N1 or EPR target setting, etc., etc.

Maybe someone can tell me why we don't use this technology. Fighter jets have been using this since before time began....


Shore Guy
10th Jan 2008, 11:54
This airspeed anomaly did not work out so well.....


Check Airman
10th Jan 2008, 14:20
In addition to all that's been said above, after a while on type, you know what powers setting, pitch, vertical speed and airspeed will look like for typical weights. If you set the power and the pitch, and adjust the vertical speed for what would be a "typical landing", unless you're in very turbulent conditions, you should find yourself right there about Vref, plus or minus a fer knots.

stator vane
10th Jan 2008, 14:41
i suspect that it will not be as easy as it sounds here on a computer screen with coffee in hand.

if at night time--poor weather. and if the same thing happens to your altimeter--i.e. taped ports--it will not be an easy task at all. all the indications would have to be ignored. one will wish that he/she had those old sticky instrument covers--to put on the faulty instruments.

add in all the warnings that might be going off behind your head-- and if you ended up in the clouds,---

it will not be an easy task at all.

10th Jan 2008, 15:42
Attitude, attitude, attitude.

Hopefully you know by now the typical pitch attitudes, and power settings for your aircraft.

Use IRS/GPS/FMC, if available, for GS. If not ask radar controller for GS info.

The chance might be billions to one but didn't it happened to an MD-11 going into EZE(Buenos Aires) years ago? Flight crew was awarded annual award for their outstanding performance.

Oh, yeah, just to make it interesting I believe it was full IFR after a 9-10 hr overnight flight. I believe weather included significant rain. All in all about as difficult as you can envision.

Check Airman
10th Jan 2008, 17:07
Stator Vane,

I wasn't suggesting it would be an easy affair, just that it's doable. If more instruments are inop though, that's a different affair, but the question specified airspeed indicators.

stator vane
10th Jan 2008, 20:52
i've never seen a double airspeed indicator failure the the sim sessions i have been in-and that is growing to be a significant number--

one side or the other and then the other pilot ends up flying it.

the 738's have the angle of attack probe, but i will have to dig into the books to see if there will be any clues after all the airspeed indicators have taken their annual leave together.

i seldom fly the flight directors on take off, though my company requires them to be on---i look through them and put the aircraft angle where the F/D's eventually come to later.

but in reality, it will be a handfull for sure.

puts more meaning on the 80 knot call out!!! surely if the sensors had been taped, we would see it at that point, if we were really "looking" rather than seeing what we want to see.

10th Jan 2008, 21:18
I used to give a standard extra time exercise on endorsement training (732/733/734) to emphasis the Flight with Unreliable Airspeed thing and for progressive I/F confidence building during the endorsement program. Worked a treat for the latter .. but, of course, relied on good pre-sim briefing and a nil-anxiety environment for the guys and gals up front.

(a) total speed and height info failure .. often the sim couldn't accomodate everything and we had to revert to covering those things which couldn't be failed by pressing buttons at the organ grinder's station

(b) T/O in restricted vis

(c) aim to fail everything desired by shortly after gear up - end up with no airspeed, altitude, VSI, radalt, etc., cues so that the crew had to rely on configuration, attitude, and thrust to continue the recovery

(d) crew flies a climbing circuit to intercept the ILS for recovery to nominal cat 1 minima

(e) check for compatible FPA/GS from the configuration, attitude and thrust numbers, considering expected wind

(f) fly it down to a landing

Most folk walked it in on the first attempt and I can't recall anyone needing more than a second look at the problem.

Not directly transferable to line operations in any sense and quite artificial ... but served a useful directed skills purpose. The guys and gals probably would never get to see a similar exercise again but the basics stay with them.

11th Jan 2008, 13:31
I have experienced quite a number of ASI failures, all caused by blockages, and all in single pilot aircraft with only a single panel and pneumatic instruments.

I have had a number of bugs in the pitot tube causing a complete blockage - zero indicated airspeed, a little disconcerting, but quite straightforward - power plus attitude equals performance, the erroneous reading was obviously incorrect.

Bugs in the pitot tube causing a partial blockage - under reading by approximately 15 - 20 knots - very disconcerting as it was very difficult to recognize and confirm especially given that it was my first flight on an aircraft with this type of wing in over a month. Got airborne, kept nose down to build speed, went past office low and fast (office is alongside the runway), set a climb attitude and climbed away at a lower than indicated but higher than normal actual airspeed. Level flight and approach reinforced the diagnosis, with the confirmation being an off the low end of the ASI scale touchdown.

The third type of failure I experienced as a pax sitting in the RHS - a static blockage. Lift off speed indication was normal, and as altitude increased the airspeed indicator started to under read. A quick glance at the altimeter which indicated airfield elevation, and VSI which indicated 0 suggested a static blockage. On opening the alternate static valve the readings returned to normal.

As to procedure - it depends. The most critical time for accurate airspeed is approach and landing - cruise and descent without an ASI is quite straightforward (VFR). With experience in an aircraft the glances at the ASI only confirm what you already know from power and attitude. Personally I would avoid landing where there is a lot of turbulence and/or wind shear. Somewhere where there are engineers to fix the problem is always handy too.

11th Jan 2008, 14:57
I know this senario must be a nightmare in the cockpit so you've lots to do but don't forget that many ATC radar centres these days have capability to display the groundspeed on the radar so can relay this. We can also ask other a/c in the sky what indicated they are flying and how this relates to their current groundspeed for conversion sake too (taking windspeed/direction into consideration). Its something I've actually simulated as a radar controller too with a professional pilot acting as the pilot in trouble and both sides learnt a hell of a lot as to how ATC can be of use. We do have many tools to play with these days..........

11th Jan 2008, 19:10
Reading through the replies, it seems that the general consensus is that to know/feel the pitch attitude and power setting. However, how come the pilots from the Aeroperu flight weren't able do that and how drastically would this change if you have an engine out?

11th Jan 2008, 21:56
I tried to resist, but I can't bear it...

You need a cat and a duck. And a window that opens.

There. I said it.