ATA 34-11, Altimeter Splits

Modern air data systems use advanced technology for sensing ram air (airspeed) and static pressures (altitude). Maintenance manuals don't go into just how a air data computer computes pressure signals, but I "believe" a internal vibrating transducer reacts to pressures in a chamber.

These systems are extremely accurate. Altimeter splits of a hundred feet or more at 35K were normal with older bellows based airspeed, altimeter, and air data systems. With digital's, 40 feet or more at 35K are uncommon.

If a crew reports an altimeter split beyond acceptable limits, the possibility of a leak must be considered.

The usual path by maintenance is to replace the suspect altimeter or air data computer first. If both those components have been tried and the problem persists, chances are....... there's a leak.

The easiest way to at least verify a leak is to tape over the static ports, close the aircraft up, and pressurize it. The suspect system will have a altimeter that starts to drive down. This doesn't find the leak, but it does verify that you've got one.

Pressurizing the line to 10psi or so and using leak detector would be my preferred method of leak isolation.

Wow, I have often wondered about this problem and how to attack it on a turn. It sounds simple, since pitot pressure doesn't have an input to Altimeter reading, don't need to block off pitot probes! I like this approach Mark!!

A pitot system can be checked for leaks by just applying pressure (simulating airspeed) to the line. If the system is tight, it should hold.

My problem over the years is not having the correct pitot tube adapter and having leaks at the rubber tube placed on the probe. It's kinda tough to verify a leak if you're inducing one of your own.

I haven't checked the Boeing's, but I do know the MD-11 has sensors right at the pitot probes and static ports. The sensors send data directly to the ADIRU's. There is no need for pitot/static lines which pretty much negates the possibilities of leaks.

I'll have to look to see how 75's and 76's are connected.