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Take off with failed airspeed indications

FMF15E

Active Member
I went out with my son for a few hours of RV fun over the long weekend. I did a complete preflight to include looking in the pitot tube (my plane is stored inside a hangar, but without a pitot cover). Start, taxi and run-up were normal, weather was clear, slightly gusty, 10 kts at 200. As I advanced the throttle on rwy 18, the airspeed indicator began to move and I checked the engine indications. The airplane was already airborne when I realized the airspeed indications were way low at 50 kts during the initial climb and full throttle. It was too late to abort the take off due to a 3000 ft runway. I have dual Grand Rapids screens up front (HX and HS) and a HS in back. I also have a back up 3" AS indicator. They all read about 49 kts in what should have been about 120 kts. I turned crosswind, declared that I would remain in the pattern and set up for landing immediately after take off. I made one low approach and then landed on the second time around after using the GPS ground speed, 8.5" MP and normal pattern feel to make a faster than normal (I think) wheel landing. We taxied back to the hangar and began to troubleshoot. I thought it might be a plugged pitot and diconnected the pitot tubing from the instruments and blew it back out to the pitot. We tried another take off, but aborted on the runway with the same problem. A full analysis showed a significant leak which was caused by cracks in the plastic block of my TruTrak Digiflight II VSGV. I understand that TruTrak has gone to a metal block now (my AP is 4 yrs old and has been flawless until now). I used nylon fittings and hand tightened the T fittings in the back, but they cracked at 300 hrs.
Glad it was a clear day. Not throwing stones at TruTrak - they have a great product and great support as well as a service bulletin on something, but I can't open it. This is just a data point and another thing to think about when you take your aerospace vehicle out for a spin.

Bruce "FM" Edwards
RV-8, 391FM, 300 hrs
 
airspeed to altimeter

Good demo of airspeed indicator acting as altimeter when lacking sufficient pitot air.

Good job landing with 3k' runway.

Lost pitot air one night flying a Piper Seneca. No GPS then, but had very long runway, so just came in fast and bled the speed off in ground effect, let it settle to runway.

Realize you had a cracked fitting, but here in South FL at least, the insects can build a nest of sorts in the pitot tube in a matter of hours. Then we also have moisture and dirt that can get in there, affect accuracy at best, freeze up at higher alt at worst, so makes the case for pitot cover always when parked.
 
I'm off to the airport today to try to track down a leak in my system. Airspeeds are reading way low. Preliminary check shows leak somewhere. I have a VSGV also, but it is new.
 
I have dual Grand Rapids screens up front (HX and HS) and a HS in back. I also have a back up 3" AS indicator. They all read about 49 kts in what should have been about 120 kts


I've written this before, but I think it bears repeating:

Any instrument critical to the safety of flight needs redundancy.

RVs have very little natural stall warning, and will depart (stall) suddenly if provoked.

Multiple indicators hooked to the same source don't provide real redundancy.

Airspeed is important because it allows us to estimate angle of attack, as long as we know our weight and g-loading. A direct presentation of AOA is even better, as long as it isn't connected to the same pitot source.

It's really easy and cheap to add a second pitot source. My RV-8 has two blade type pitot tubes, one on each wing. One feeds the ASI (s) and the other feeds the AOA system.

A self-contained system like the LRI is cheap and works really well, and is pretty quick to install.

A stall-warning vane like the RV-10 is pretty easy (really just a crude, single point AOA) and works well on gazillions of certified airplanes.

Bernie Kerr put a pitot source in the back of his engine cooling plenum, and found it accurate within a couple of percent over the entire speed range--easy, quick, cheap and low-drag.

I know somebody will read this and have to point out that a real pilot can tell by the seat of his pants if he is about to stall, and airspeed indicators are for sissies. The accident statistics say otherwise, pretty eloquently I think...
 
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