danialanwar
Posts: 420
Joined: Fri Mar 16, 2001 6:13 pm

Altimeter Accuracy

Fri Jul 26, 2002 12:21 pm

Does anybody have statistics on altimeter accuracy. That is, if the altimeter reads (say) FL360, how likely am I to really be at that altitude (or more precise, at that air-pressure-altitude) and how likely am I to be off? I guess hwat I'm looking for is a mean divergence (hope to be zero) and a standard deviation / variance, plus preferablytype of distribution (is divergence normally distributed?)

Thans!
Daniel
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JETPILOT
Posts: 3094
Joined: Tue May 18, 1999 6:40 am

RE: Altimeter Accuracy

Fri Jul 26, 2002 5:18 pm

One of the preflight checks is to check alitmeter accuracy with the current field alitmeter setting.

Must be within plus or minus 100ft of actual field elevation.

JET
 
PW4084
Posts: 287
Joined: Tue Mar 06, 2001 7:31 pm

RE: Altimeter Accuracy

Fri Jul 26, 2002 9:53 pm

JET, Isn't the FAA standard within 75 feet of field elevation? In that case dual altimeters could potentially be 150 feet apart and not require adjustment.

PW4084
 
broke
Posts: 1299
Joined: Wed Apr 24, 2002 8:04 pm

RE: Altimeter Accuracy

Fri Jul 26, 2002 9:57 pm

The accuracy of the entire altimetry system (Altimeter, Air Data Computer, Static Ports and Lines) must be no more than +- 135' for an airplane to qualify for operation in RVSM airspace. Prior to that, the system error had to be no more than +- 260'. This is for flights from FL 290 to FL 410.
 
JETPILOT
Posts: 3094
Joined: Tue May 18, 1999 6:40 am

RE: Altimeter Accuracy

Sat Jul 27, 2002 12:20 am

Sorry 75 ft.....

No more early morning posting for me.

JET
 
rendezvous
Posts: 531
Joined: Sun May 20, 2001 9:14 pm

RE: Altimeter Accuracy

Sun Jul 28, 2002 4:29 pm

In the standard atmosphere, then air pressure drops 1hPa every 30' you go up, so altimetres are callibrated to show that change. The thing is though, on a hot day 1hPa could actually be a 35' increase in altitude because the air is less dense, or on a cold day 25' per hPa.

I imagine by FL360 there could be some substantial error, however, all altimetres will have the same error, so it doesn't mean planes are going to collide.
 
danialanwar
Posts: 420
Joined: Fri Mar 16, 2001 6:13 pm

RE: Altimeter Accuracy

Tue Jul 30, 2002 4:09 am

Thanks guys. I think I can work with these numbers. Great people on this forum!
Best Business Class: Royal Brunei. Best Economy: Singapore Airlines. First: please send money first!
 
POSITIVE RATE
Posts: 2121
Joined: Sun Sep 16, 2001 11:31 am

RE: Altimeter Accuracy

Wed Jul 31, 2002 10:48 pm

What is the altimeter accuracy of high flying aircraft such as Concorde? What about other experimental a/c like the X-15 flying at 300,000'+?
 
wardialer
Posts: 1139
Joined: Fri Sep 14, 2001 1:08 pm

RE: Altimeter Accuracy

Thu Aug 01, 2002 12:11 am

The rule is, your suppose to have 50 to 100 feet lead. For example when your at FL360 and your altimeter reads 35900 or 35950 thats OK. Altimeter readings are not always accurate because of such factors as wind, turbulence, and temp.
 
JohnM
Posts: 379
Joined: Sat Feb 24, 2001 12:35 pm

RE: Altimeter Accuracy

Thu Aug 01, 2002 8:55 am

If memory serves, the limits for what I work are:

10,000 ft +or- 50 ft
29,000 ft +or- 75 ft
41,000 ft +or- 100ft

These are the RVSM limits that came out a few years ago. Almost every time, the actual altimeter indicator readings are about plus or minus 15 feet. The CADC is very accurate, as long as it is working. Most altitude errors that I find are static system leaks, not CADC problems.
 
Guest

RE: Altimeter Accuracy

Fri Aug 02, 2002 8:26 am

According to our TCAS readings in European RVSM airspace, altimeters seem to be very accurate, "ours" or "theirs"... when crossing traffic above or below, we consistently read 900 to 1,100 feet vertical separation... and that is for levels such as 390...
(s) Skipper
 
apathoid
Posts: 887
Joined: Fri Sep 28, 2001 3:19 pm

RE: Altimeter Accuracy

Fri Aug 02, 2002 8:35 am

AC 43-13/1B Appendix E lists altimeter accuracy and acceptable leakage rates for American certified aircraft. Look it up.

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