Hank Posted October 5, 2013 Report Posted October 5, 2013 How long does your altimeter last between OH/exchange? I bought my Mooney in 2007, failed pitot static in 2009 (ASI & Alt), and again this week (Alt only). Should I expect altimeter failure every 4 years, or could it be related to my vacuum failure this spring?
Piloto Posted October 5, 2013 Report Posted October 5, 2013 Altimeters by the their simplicity nature are very reliable. Never had one fail on me in over 40 years. Some altimeters are encoders also and this is the part that is prone to fail. If you are having altimeters failures I would check the instrument panel rubber mounts. The rubber mounts dampen the engine vibrations to the instruments. José
Bob_Belville Posted October 5, 2013 Report Posted October 5, 2013 My avionics shop told me panel mounting rubber is no needed. In fact a little vibration is helpful to keep certain "needles" from getting stuck. I suppose that might not apply to every older device, we were talking about Aspen, GTN and JPI EDM specifically.
manu damaschin Posted October 5, 2013 Report Posted October 5, 2013 Are the other instruments based on static pressure working well? Could it be a static system leak or blockage? The altimeter does nothing to do with the vacuum...
jetdriven Posted October 5, 2013 Report Posted October 5, 2013 My avionics shop told me panel mounting rubber is no needed. In fact a little vibration is helpful to keep certain "needles" from getting stuck. I suppose that might not apply to every older device, we were talking about Aspen, GTN and JPI EDM specifically. My DG precesses a lot less after I changed the shock mounts out. But Bob is right, our altimeters in larger airplanes actually have vibrators installed that keep the altimeter from sticking.
triple8s Posted October 5, 2013 Report Posted October 5, 2013 Why have an "Encoding" altimeter insead of a seperate encoder? Whats the benefit?
takair Posted October 5, 2013 Report Posted October 5, 2013 How long does your altimeter last between OH/exchange? I bought my Mooney in 2007, failed pitot static in 2009 (ASI & Alt), and again this week (Alt only). Should I expect altimeter failure every 4 years, or could it be related to my vacuum failure this spring? Hank Did they tell you the failure mode? Was it just slightly out of calibration? The shop should be able to bring that in, if they are rated for it. Does the shop use a digital pitot/static test set? They are far more reliable and cause fewer false altimeter failures. The altimeter should not fail every 4 years. Mine might actually be the original.
Fly By Night Posted October 5, 2013 Report Posted October 5, 2013 Why have an "Encoding" altimeter insead of a seperate encoder? Whats the benefit?
Fly By Night Posted October 5, 2013 Report Posted October 5, 2013 As I understand it, an encoding altimeter is required for the Altitude Capture mode of your autopilot to function.
jetdriven Posted October 5, 2013 Report Posted October 5, 2013 Why have an "Encoding" altimeter insead of a seperate encoder? Whats the benefit? I'm not sure if there is a benefit, other than the autopilot feature. They have a single-point failure mode built in. Let me explain. I read an article where the pilot's altimeter indicated 5,000' on a night IFR X/C. Approaching the destination, he got 3,000' and after nearly missing some obstacles, managed to find the airport under VMC and landed. Center and the altimeter showed him at 5,000' enroute, but he was a whole lot lower. A blind encoder would have been a second independent source of altitude to ATC. But the encoding altimeter reports what the needles show.
flyboy0681 Posted October 5, 2013 Report Posted October 5, 2013 While on the subject, the glass on my altimeter is terribly fogged over (but still readable). Is the unit sealed or can the glass be safely removed and cleaned without affecting the innards?
jetdriven Posted October 5, 2013 Report Posted October 5, 2013 You can have it OH'd for 225$ here. http://www.flyaqi.com/pressure.htm
Piloto Posted October 5, 2013 Report Posted October 5, 2013 Why have an "Encoding" altimeter insead of a seperate encoder? Whats the benefit? Back in the late 70s when Mode C was made a requirement encoding altimeters were not as compact or as inexpensive as they are now. A blind encoder was an actual altimeter mechanism with internal encoded discs instead of needles that were used to generate the gray code. Later on to minimize cost the encoder discs were integrated in a regular altimeter. With the development of microcontrollers and pressure transducers the altitude encoder could be package in a small box and sold at a lower price. The only advantage of the encoding atimeter is the no need for a separate box. They are very reliable and if they are working good there is no need to have it replaced with a modern encoder. José
N601RX Posted October 5, 2013 Report Posted October 5, 2013 There are 2 things that they check on a altimeter. One is the accuracy and the other is they check it for leaks. If the leak is around the glass, the shop who does mine fixes it with a coat of clear nail polish around the edge of the glass (The FAA has approved this). During the accuracy check they put it on a vibration table that makes it sensitive to small changes.
pirate Posted October 5, 2013 Report Posted October 5, 2013 My altimeter lasted 18 years. I replaced this past July during a IFR check ( leaking ).
N201MKTurbo Posted October 5, 2013 Report Posted October 5, 2013 While on the subject, the glass on my altimeter is terribly fogged over (but still readable). Is the unit sealed or can the glass be safely removed and cleaned without affecting the innards? Most instrument shops will clean the glass and put in a new gasket and re-certify it for less then a hundred bucks. I would suggest doing it during your next pitot static cert. The hanger fairies have been known to do it.
Hank Posted October 5, 2013 Author Report Posted October 5, 2013 My A&P said it was borderline on the test, but "because you actually fly IFR," he replaced it. Better safe than sorry. Four years ago he exchanged it for an OHed unit, I'm supposing that's what he did this time. As long as he leaves my IVSI alone!
rbridges Posted October 5, 2013 Report Posted October 5, 2013 my altimeter had to be OH'ed this month; too much discrepancy at 20K feet and it failed the friction test. my avionics guy said it should be good for a long time in his experience. Hank, I think you just got bitten by some bad luck.
takair Posted October 6, 2013 Report Posted October 6, 2013 My A&P said it was borderline on the test, but "because you actually fly IFR," he replaced it. Better safe than sorry. Four years ago he exchanged it for an OHed unit, I'm supposing that's what he did this time. As long as he leaves my IVSI alone! Hank Your A&P likely has a repair station rating to do the test, but not to make the adjustment. He was likely correct in failing it, but it was likely a simple 5 minute adjustment by an instrument repair station. When I ran a shop, we ran into this most often before we went to a digital test set. The problem with the older analog test sets is that they are as likely to drift as your altimeter and take more of a beating. I found we needed to do a test set calibration every 6 months to the analog box, annual for the digital. If this happens again in the next 10 years be sure to question it and ask if it can be adjusted. There are other areas, like friction and hysteresis that can not be adjusted.
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