Altimeter Accuracy

[Jim Stewart]

Here's the required tests:

http://www.flightsimaviation.com/data/F ... -appF.html

Note that it is a transponder test and nothing more. It tests receive sensitivity, timing, frequency accuracy and transmit power. Nothing in the regulation requires testing the altitude encoding.

[zdc]

You paid more (and got more) than the FAR's require.

That could well be.

They don't check the altitude accuracy other than the reported airport altitude during the test.

Gee, I wonder how that could possibly satisfy ATC's Mode C requirements? Or don't they care if your indicated altitude is really correct??

As for VFR aircraft, ATC could care less what altitude the pilot thinks he is at, the controller just wants to know that the mode C altitude associated with a target is accurate. The pilot has a responsibility as regards to the indicated altitude. If a VFR pilot commits a violation due to an inaccurate altimeter, it is on the pilot even though regulations don't require a pitot/static check.

[drseti]

As I've stated elsewhere, rather than arguing legality, I'd rather discuss practicality and safety. (I'm not an attorney, and if I were, I'd probably not admit it on this forum. )

Sure, an aircraft owner or FBO could save $125 every 24 calendar months by doing only the minimalist transponder test on VFR aircraft. But I believe that's false economy.

My plane flies regularly in Class C and B airspace, underneath layers of B and C airspace, within 50 miles of the Washington DC SFRA, and sometimes on VFR flightplans within the SFRA. In such airspace, even if VFR, the clearances are very IFR-like. Assigned altitudes are binding, and Mode C accuracy becomes important. Let's say somebody busts restricted airspace because the pitot-static system hadn't been checked. Or, maybe they didn't bust restricted airspace, but because the transponder codes were wrong, ATC says they did. How much lawyer time do you suppose that $125 will buy them?

But even outside a radar environment, remember that TCAS interrogates transponders, and determines separation from Mode C returns. It isn't hard to imagine an accident caused by an inaccurate encoder (if someone wants to make this a research project, there's probably a doctoral dissertation in it somewhere, and plenty of NTSB and ASRS data to be mined). An erroneous Mode C code could trigger a resolution advisory in a nearby TCAS-equipped aircraft, resulting in improper avoidance maneuvers, possibly climbing or descending an aircraft into a target that was not previously a conflict. How would the FBO then feel about the $5.21 a month saved by not doing the full check?

[zdc]

As I've stated elsewhere, rather than arguing legality, I'd rather discuss practicality and safety. (I'm not an attorney, and if I were, I'd probably not admit it on this forum. )

Sure, an aircraft owner or FBO could save $125 every 24 calendar months by doing only the minimalist transponder test on VFR aircraft. But I believe that's false economy.

My plane flies regularly in Class C and B airspace, underneath layers of B and C airspace, within 50 miles of the Washington DC SFRA, and sometimes on VFR flightplans within the SFRA. In such airspace, even if VFR, the clearances are very IFR-like. Assigned altitudes are binding, and Mode C accuracy becomes important. Let's say somebody busts restricted airspace because the pitot-static system hadn't been checked. Or, maybe they didn't bust restricted airspace, but because the transponder codes were wrong, ATC says they did. How much lawyer time do you suppose that $125 will buy them?

But even outside a radar environment, remember that TCAS interrogates transponders, and determines separation from Mode C returns. It isn't hard to imagine an accident caused by an inaccurate encoder (if someone wants to make this a research project, there's probably a doctoral dissertation in it somewhere, and plenty of NTSB and ASRS data to be mined). An erroneous Mode C code could trigger a resolution advisory in a nearby TCAS-equipped aircraft, resulting in improper avoidance maneuvers, possibly climbing or descending an aircraft into a target that was not previously a conflict. How would the FBO then feel about the $5.21 a month saved by not doing the full check?

You express a lot of concern about mode c accuracy,but that is not the issue here because your xpndr must be checked every 24 months. Other than a mode c verification from ATC it is impossible for a pilot to know without a test whether his xpndr is functioning properly. However, the accuracy of the altimeter is easy for a pilot to check on the ground and in the air. If I'm not having any pitot/static problems I would prefer to save $125 and not run the risk of a mechanic who got a little careless and damaged the system . The pitot/static system is not something that requires preventitive maint. If the test shows a problem you as a pilot should have already been aware of it and had it repaired. In my opinion it is one of those things that can be left alone until it needs repair. So it is not false economy at all.

[drseti]

Other than a mode c verification from ATC it is impossible for a pilot to know without a test whether his xpndr is functioning properly.

Actually, there is another way. Are you familiar with the Zaon MRX personal collision alert system? This 3/4 x 2 1/2 x 4 1/2 inch box weighs 5 1/2 oz (including batteries). In addition to decoding other aircraft's Mode C transponders, it receives and decodes your own transponder signal. (By comparing your Mode C readout with a target's, it determines relative altitude). At the push of a button, you can make it briefly display your own transponder code and Mode C altitude. Sure, it doesn't satisfy any FARs, but with this little box sitting on the glareshield, I can verify my transponder and encoder on every flight, if I so choose (this in addition to its intended function as a traffic warning device).

I have no commercial affiliation with Zaon, and am recommending their product purely on the basis of personal experience over the past few years. Those of you who know me will be aware that I am the inventor of BiDCAS (a TCAS predecessor, circa 1980s). When first I saw the Zaon, I was blown away. I am arguably an expert on collision avoidance technology, and I couldn't have done it myself, this well and this cheaply.

[zdc]

I do have a ZAON alert system. I have the one that is smaller than a pack of playing cards and runs on AA batteries. I regularly check the ZAON against the xpndr. Other than an occasional false alert, I have been very happy with the product. I spent $400 for the unit with no installation costs, and in my opinion it is better than TIS systems that cost thousands because a ZAON will work anywhere in the country unlike a TIS.

[drseti]

I do have a ZAON alert system. I have the small one that is smaller than a pack of playing cards and runs on AA batteries.

Yep, that's the one. I'm delighted with mine, and am glad to hear that your experience with it has also been positive. (You got a bargain at $400. Mine was $549, I believe, when they first came out...)

[3Dreaming]

Question: When you set your altimeter to Barometric pressure according to ATIS or other accurate info service, what is the general deviation you see on the altimeter vs. your known altitude? I'm curious because I noticed >100 ft variations while doing ground preps the other day on the aircraft I fly. The handbook of aeronautical knowledge says variations of >75 ft are not acceptable.
I also notice a considerable deviation from GPS altitude.
So, I guess my question is: should a properly barometrically adjusted altimeter be showing relatively the same thing as the GPS? My thought would be yes (within 50 ft or so).

Thanks,

Target

In one of your other post you said you are flying a Sportstar. Are you having problems with a Dynon unit, or a analog gauge. A analog gauge will require some vibration to overcome the friction in the instrument. If you are turning it to the altimeter setting while just sitting there it may be off a bunch. If it is the Dynon it is very easily adjusted in the unit itself, and someone may have changed it accidently. Tom

[zdc]

Well, when you get down to it, I wouldn't pay for a xpndr check unless I had to. The only thing the test does is tell you that the xpndr is functioning properly at the time of the test, it could fail the next day. It's one of those things that works until it doesn't and fails without warning.

As Mr. Stewart pointed out, a modern xpndr malfunction won't cause havoc in the system like the old xpndrs would. I believe the reg could be ammended to allow pilot checks of the xpndr, like VOR checks that don't require expensive tests.

[drseti]

Well, when you get down to it, I wouldn't pay for a xpndr check unless I had to.

From the perspective of the owner/pilot, I can certainly see that. As an FBO, I have to approach things a bit differently, since there are liability issues to consider. It's surprising how much CYA activity this flight school business involves!

[zdc]

Just as a side note, I've owned the same airplane for 25 years. I'm now into the seventh year of my third xpndr. Not once, has my xpndr been adjusted/repaired/replaced due to the result of the mandatory 24 month xpndr check. The results always were PASS. The unit would fail/malfunction in between tests. I knew the unit wasn't working one of two ways: No interogation being displayed, or ATC told me there was a problem. If a VFR pilot has no inter action with ATC and doesn't bother to do some kind of self initiated check, he could fly blissfully in between checks with a bad xpndr.

While the mandatory xpndr check is a good thing for avionics shops, I consider it wasted money.

[drseti]

Just as a side note, I've owned the same airplane for 25 years. I'm now into the seventh year of my third xpndr.

I did a little better than that. My last airplane I owned for 30 years, and got by with the same transponder all that time. Of course, I did have it repaired twice. Once, I had to replace the cavity oscillator - expensive! Modern solid-state transponders tend to be far more reliable.

[zdc]

Your experience with xpndrs is about the same as mine, except I opted to replace instead of repair. When you have a xpndr, radio or some other piece of electronic equipment go bad, it's a crap shoot as to repair or replace since labor and parts can be expensive. You could repair for less than new and perhaps the repaired unit will last just as long as a new unit, or you could repair only to have a different component fail shortly after repair. And since you just invested money in a repair you opt again for repair but now you have exceeded the cost of a new unit. Avionics are expensive enough, so I would just as soon forgo a costly mandatory test that has no value if I could. But, it is a regulation so I have the check done and pay for it.

[zdc]

Yep, that's the one. I'm delighted with mine, and am glad to hear that your experience with it has also been positive. (You got a bargain at $400. Mine was $549, I believe, when they first came out...)[/quote]

I bought mine about a year ago and I got lucky on the price. At the time the advertised price was about $549. I was on line looking for something at "Aircraft Chief" and they had a one week special for the small unit for $400 and free shipping. I jumped on it. I sometimes wish I had bought the more expensive unit [that requires installation] that you can attach to a 496 and the traffic appears on the 496. Either way, this is cheap technology that doesn't require a subscription , is well worth the cost and I highly reccomend the product.