Bad Altitude — A Formula for Disaster

While talking once with avionics professionals at the Aircraft Electronics Association show I learned some problems they found over the course of the years — one of which was so serious I nearly dropped my flight bag.  The issue involved “paper” altimeter static checks, or the routine check that allows ATC to be sure that your plane is at the altitude it says it is.

It seems there are unscrupulous individuals are driving around to airports, and offering static checks out of the trunks of their cars.  They claim that they check the altimeter and static system of airplanes, and charge owners about half of what an avionics shop would.  The problem is simple: some of these guys aren’t really doing anything other than fiddling around, signing the log books, and taking the pilot’s money.

The Static Check is required by the FAA, and makes sure your altimeter and altitude encoder are both working on the same level.  The shop hooks up to the static system of the airplane, and runs the “altitude” up and down, while watching what the transponder, encoder, and altimeter are reporting.  Any problems are then resolved through calibration.

Believe it or not, the static system of your airplane can get out of alignment.  The altitude encoders that report your altitude are full of electronic components that can wear out — heck, I had to replace mine not too long ago after it wouldn’t calibrate properly.  Altimeters on the other hand, can get out of whack due to “hysterisis,” which is a fancy, avionics word that means the indicator is sticky in certain places.  Problem: If the little pointer stops when you don’t, or it won’t hold the right calibration, you might think you are somewhere you’re not … and you might not be the only one.

You may think there is plenty of space in the air as you fly VFR between here and there.  However, as you blissfully fly through the air, you are passing near other aircraft on a routine basis.  If the area is full of convenient airports you’ll be sharing space with a lot of aircraft.

ATC is depending on your altimeter and encoder to give them the right information as they choreograph movement in the sky.  Let’s paint some possibilities…

You are flying VFR with a paper static check.  Unknown to you, your altimeter is reading 300 feet low, and your encoder is reporting 200 feet high. Being a good VFR pilot, and flying east, you pick an altitude of 3500 feet.  Unfortunately for you, this means you are really flying at 3800 feet, and your altitude is reporting 3600 feet. A westbound pilot on an IFR flight plan is flying at 4000 feet and closing with your airplane.  ATC informs him of VFR traffic, 6 miles, 12 o’clock, opposite direction, at 3600 feet unverified, and our IFR pilot starts looking.

As our IFR pilot leans forward to look around, he unknowingly pushes on the yoke (come on — it’s never happened to you, has it?) and his altitude starts to drop … just a little.

Note: Closing at 300 knots puts the two aircraft well under two minutes apart. Is that enough time for either pilot see (and potentially avoid) the other? The VFR pilot doesn’t even have a clue.

DON’T BE THE PILOT OF A PAPER AIRPLANE!  Know what maintenance is being performed on your airplane.  If you are familiar with what a static check entails and elect to allow someone who works out of their trunk to work on your plane, STICK AROUND AND MAKE SURE IT IS DONE RIGHT.  If you aren’t familiar, then we strongly recommend that you visit a reputable avionics shop for the work.  Airplanes aren’t made out of paperwork, but anyone who signs off improperly documented maintenance on your plane is going to get you burned!

 Liner Notes: The shops that observed these ‘paper‘ inspection activities have appropriately reported their findings to the FAA, and an investigation was put in progress.  iPilot does not mean to imply that anyone who works out of his or her trunk is disreputable.  However, unless you are intimately familiar with the person doing the maintenance, or are familiar enough with the task to know whether the work you are observing is adequate, you are taking a big risk for yourself, your passengers, and maybe even more…