Jun 14, 2009

Lies, Damn Lies and Instruments...

After reading Aviatrix's excellent summary of an Air Caraibe incident related to 'bad' air data and instrumentation malfunctions, I was shocked to discover the following information from a Boeing website:


During the past 10 years, more than 300 accidents and incidents have been reported as a result of erroneous flight deck information, including problems with pitot-static probes and air data computers. Several fatal accidents that involved erroneous flight instrument information and six incidents resulting from lost or erratic air data occurred in 1996 alone.

Some fatal and near fatal accidents have been tracked back to something as mundane as a strip of tape over a sensor port.  The Air Caraibe incident involved severe and sudden icing in cruise flight, and may prove to have some common elements with Air France 447.

To a pilot who has become finely tuned and trained to flying by instruments, suddenly having to determine which pieces of conflicting information are right and wrong and how to respond is a huge challenge.  During flight in turbulence at high altitude the margins for error can be very small (see previous posts about airspeed envelopes.)  A wrong decision and control input can quickly put a jet airliner into a situation from which there is little chance of recovery.

Understanding the causes behind Air France 447's demise is extremely important - making it even more urgent that the 'black boxes' are found.

3 comments:

david said...

Someone once pointed out to me that for a B747 or A330, every takeoff is a short-field takeoff, and every landing is a short-field landing.

Imagining what it would be like to operate my Cherokee every day from a 1,000 ft grass strip helps me realize how important airspeed control is for the big planes.

Anonymous said...

I'm curious to know if there has been any development in creating a different way of measuring IAS. It's surprising that such an old technology with so many possible issues is used these days.

Any known developments in that sector Alu?

-Jay in YOW

david said...

Jay: that's a really good question.

As you probably know, what we measure as IAS isn't really a speed -- it's the difference between static pressure and ram air pressure. It would be more accurate to talk about IAS in terms of inches of mercury or millibars rather than knots.

With that in mind, is there a better way to collect air for measuring static and ram pressure than a tiny hole facing sideways (for static pressure) and a tiny tube facing forwards (for ram-air pressure)?

On my Cherokee, I've lost my ASI twice already due to a blocked pitot system (once in low IMC), so I know the current arrangement is far from reliable.