View Full Version : Airspeed Disagree

11th May 2002, 20:41
Im not a real pilot but I am a soon to be one. I have managed to get my hands a few boeing manuals and when reading one for the 737-700, I came across airspeed disagree. My question is what causes it, does it happen often, and what, if any pilot action should be taken and if any pilot action can fix the problem?
Jon Gibbs

11th May 2002, 22:54
The way it works is that the Capt and F/O have different air data sources (i.e. pitot/static ports). One of those may be blocked (by a tape for instance) and cause errorneous airspeed indications on one of the airspeed indicators, while the other shows good values. The problem is that it may sometimes be difficult to recognise which one is good, which, since the airspeed is vital for safe flight, may be fatal. Actually - at least two airplanes (both were Boeing 757's) have stalled and crashed because of errorneous airspeed indications. Now, on most airplanes pilots should compare the speed indications at certain speed (usu 80 Kts) during takeoff run, and if there's a disagree - they must abort. In the air, if this happens, the pilots should fly manually, using typical power settings and attitudes and identify the wrong indicator (eg. by comparing to the third - standby indicator) and use the good one.

Hope I explained it well

11th May 2002, 23:03
The pitot and static sense probes and ports are heated, and the failure of the heater elements can create icing which also gives airspeed errors. At least one 727 was lost due to this.

12th May 2002, 03:11
Actually, tinyrice, it wasn't a heat failure...it was a failure to turn it on in the first place. NW, I believe, on a ferry flight near HPN.
If one disregards the checklist, the end result can be...not good.

12th May 2002, 04:52
so much for trying to be tactfull...................

Shore Guy
12th May 2002, 11:03
The most recent accident that I am aware of where this was a player was the MD11 accident at Subic Bay. Initial reports suggested there was a discrepancy between Capt. and F/O airspeed indicators and they both switched to the erroneous (fast) source. Landed long, very hot, and went off the end of the runway at a high rate of speed. Fortunately, no one was hurt, but the aircraft was destroyed.

Photos at: http://www.airdisaster.com/cgi_bin/view_details.cgi?date=10181999&airline=Federal+Express

oxford blue
12th May 2002, 14:03
I haven't read the accident report on the Subic Bay accident, which would give the full story but, just reading the last post, I still think that, despite the extremely unfortunate outcome (which must have taken a lot of justification to management!), they still made the right decision. By taking the fast ASI for landing, they risked landing fast which, in the worst case, merely meant that they ran off the end. If they'd decided to believe the slow ASI, instead of running off the end, they could have finished up half a mile short of the runway, upside down, spinning in, as a result of a stall.

On balance, they got it right (unless there were other clues)!

12th May 2002, 19:48
Discovered a blocked pitot in a C172 2/3 the way down a 1600' runway with a ravine at the other end and decided that the wings would work better than the brakes at that time.

Flew around a bit and discovered that the A/S had become an altimeter. And the sun was on the way down. Decided to make one landing attempt on the short runway before heading over to the 9000' job at the big airport 15 miles away. Made one of my better landings.

Also spent considerable time in a Viscount simulator (no visuals) with a homicidal instructor. The only two times I cracked up were when he failed my airspeed on approach.

Much in agreement with better too fast off the other end than a smoking hole short of the threshold.:eek:

Flat Side Up
13th May 2002, 00:46
Hmmm, That MD11 story.
Correct diagnosis is vitally important BEFORE any alternate data sources are selected. All airliners are equipped with a Standby Airspeed Indicator as well as the Capt and FO sources. As well, the Standby source is from basic raw data static and dynamic pressures. If a comparison was made with the Standby Airspeed it should be apparent which source is in error. However if alternate switching by one or other of the crew members occurs before this comparison is made then the crew is left with both Capt and FO airspeeds in agreement and depending on which source was selected both the CAPT and Fo Airspeed indications could be correct or both in error.

Shore Guy
13th May 2002, 08:22
Flat Side Up and all,

In addition to the standby (raw) airspeed indicator, there are (I believe) three IRS units with GS info on the MD11. Also, there is the possibility of getting a GS readout from ATC, and if all else fails, perhaps even assistance from another aircraft (if good VFR) - lose side by side formation to confirm airspeed.

If anyone has any further info on the Subic Bay accident, could they please post it here? Thanks.