stick shakers and static port
 

I was watching a re-run on t.v. last night about the AeroPeru 603 crash, where the static ports had been taped over.

The crux of the situation was the conflicting messages that the pilots were receiving from the stickshaker and the airspeed indicator.

Does(did ?) the stickshaker in the 757 have any connection to the static port? If not, how does it work?

It's always easy to second guess, 20/20 hindsight and all that, but wouldn't the ADI have been working properly despite any problems with the static ports ?

AFAIK, a 757, C172 are not *that* different, if at all, and the gyro instruments would not be affected at all in a 172 by a static port blockage. Therefore ... pitch+power=performance ??

Mike

"Does(did ?) the stickshaker in the 757 have any connection to the static port?"

Not directly. The static ports are attached to the Air Data Computers... and the Air Data Computers feed the Stall Warning Computers... and the Stall Warning Cards operate the stickshaker.

Amongst many other things, the Stall Warning Computers need CAS, TAS and mach. These are provided by the Air Data Computers. Pitot-Static sensing elements are needed for the Air Data Computers to compute these.

Regards.
Q.

Qavion,

Ok I *thought* that the a.d. computers might be connected to the static port.

In that case, I wonder how accurate the stick shaker would be with taped over static ports ?

As for the ASI, what I've read is that it will "under Read" in a climb, "over read" in a descent. Nothing I've read has said specifically by how much the under/over read will be, or how long it actually takes to "normalize" in level flight.

Mike

"In that case, I wonder how accurate the stick shaker would be with taped over static ports ?"

Sorry, Mike, I'll have to leave this one to the physicists and design engineers to comment on (I only fix 'em) ;) Also, Boeing probably keep the finer details of air data signal processing in the Stall Warning Computer to themselves.

A good engineer/aircraft cleaner should leave a long, red (or other highly visible colour) streamer attached to any temporary pitot static system cover.

Regards.
Q.

It rather depends on the design of the system.

If the shaker is altitude compensated, then loss of accurate altitude info will have an effect.

If Mach compensated there will be another, differing effect, as the Mach will be in error also.

If purely aoa driven in theory there will be no effect unless

the sps relies on adc info to 'go live' - some systems are disabled at low speeds, as the aoa vanes are unreliable at low q. In that case the failed/affected adc may cause the spc to also fail in sympathy.

I don't know the specifics of the Boeing system.