View Full Version : Light aircraft static altimeter max permisible tolerance on ground?


Captain Punishment
16th Jul 2007, 02:08
I am after a definitive answer to a dispute that I have running with the guy who runs my flying group, if anyone could possibly help?

Both altimeters were under reading by 100ft on the ground before the aircraft went into a recent annual and C of A, it has came out the same, still under reading by 100ft on the ground. (surely this a should be calibrated as a matter of course during C of A regardless of me asking especially for it to be done??)

I asked for the altimeters to be calibrated as I feel that they are too far out. I was under the understanding that anymore than 50 or 60ft is unacceptable and am not at all happy that they are in error by 100ft.

The guy who runs the group, "in his opinion" (not an engineer by any stretch of the imagination!) sees this 100ft deviation as perfectly acceptable and he will not contact the maintenance company to determine if they have calibrated the altimeters nor will he put the aircraft back in to have it looked at as it does not see it necessary.

I have emailed CAA SRG (no reply!) to see what is the max permissible tolerance for a light aircraft altimeter so I can add fact / regulation to my argument.

I am the only guy in the group that is certified to fly IFR and the group runner feels that it is ok for him and other members to fly around VFR with 100ft under read on the altimeter. I don't feel that anymore that 50 or 60ft is acceptable.

The error of 100ft on the ground will increase with altitude and become more of a problem with separation etc at higher levels.

Any comments on this would be very much appreciated, along with possibly a technical doc / article / web link to support what tolerance is acceptable as per regulations



Pilot DAR
16th Jul 2007, 02:44
Hi Capt,

Though Canadian, this standard should give you a starting point....

http://www.tc.gc.ca/civilaviation/RegServ/Affairs/cars/Part5/Standards/a571sb.htm

Try Table 1

There should be a corresponding table referenced in your local authority's maintenance standards. Your maintenance facility should be easily able to find it.

Cheers, Pilot DAR

IO540
16th Jul 2007, 07:59
Under FAR Part 43 (basically, private flight of N-reg planes) the max allowed error is

1000ft or below - 20ft
1500ft - 25ft
2000-3999ft - 30ft
4000ft - 35ft
6000ft - 40ft
8000ft - 60ft
10000ft - 80ft
12000ft - 90ft
14000ft - 100ft
etc

bose-x
16th Jul 2007, 08:42
The common accepted error is 50ft and for IFR they must be with 50ft of each other.

Mine are calibrated every annual and rarely exceed a few of feet deviation from each other and the reference height.

But unless the aicraft is being used for IFR then I would not be worried about 100ft. You can see out the window can't you!

fireflybob
16th Jul 2007, 10:48
What type of aircraft is it?

Most flight manuals on PA38/PA28 type aircraft list the max pressure error correction as plus or minus 50 ft.

That said there are other factors. The Altimeter check should always be done on QNH whilst parked on the apron. Apron elevations are listed in the UK AIP for major UK aerodromes. Without listing all the arguments if you are using QFE there may be other "errors" as you may or may not be using a threshold QFE or aerodrome QFE and not at the "reference" point.

Also when the pressure is measured it is always corrected down to the nearest millibar. E.g measured pressure 1000.9 mb, pilot is passed 1000 mb - in extremis this means a .9 mb error which equals 25 ft (1 mb = 27.3 ft). Hence the quoted of limits of -75/+50 ft (the extra 25 ft on the negative side to cater for the possible .9 mb difference).

If you have more than one altimeter then EACH altimeter should be within the tolerance so you might have one altimeter on the minus 75 ft limit and the other on the plus 50 ft limit, hence a difference of 125 ft!

Hope this helps!

Captain Punishment
16th Jul 2007, 12:39
Aircraft is an AA5

fireflybob
16th Jul 2007, 13:15
Aircraft is an AA5

Max Pressure Error Correction almost certainly +/- 50 ft (in POH or Flight Manual)

Keef
16th Jul 2007, 13:27
So you fly happily round under the London TMA at 2450 feet (because you can) and get busted by the CAA for being inside the TMA (which starts at 2500 feet). Your excuse: the altimeter was under-reading by 100 feet?


50 feet, I learned when I did my PPL, is the maximum permissible error. If it's outside that, I'd argue the aircraft isn't airworthy. I certainly wouldn't fly it anywhere near controlled airspace on my way to have it seen to.

slim_slag
16th Jul 2007, 13:32
Anybody got a reference for 50ft? Heard it before, used it myself, never seen it in writing.

I have a reference for 'in the order of' 75ft. AIM 7.2.3.a.3

Note the variation between the known field elevation and the altimeter indication. If this variation is in the order of plus or minus 75 feet, the accuracy of the altimeter is questionable and the problem should be referred to an appropriately rated repair station for evaluation and possible correction.



http://www.faa.gov/airports_airtraffic/air_traffic/publications/ATpubs/AIM/Chap7/aim0702.html



Then, if both altimeters are misreading by 100ft, perhaps the field elevation is wrong?

fireflybob
16th Jul 2007, 13:43
So you fly happily round under the London TMA at 2450 feet (because you can) and get busted by the CAA for being inside the TMA (which starts at 2500 feet). Your excuse: the altimeter was under-reading by 100 feet?


Personally I would not feel comfortable flying that close to CAS anyway! Even in smooth conditions I would be allowing in the order of 200 feet at least when aiming to stay clear of the TMA etc! I believe there is a current CAA AIC which comments on this.

OpenCirrus619
16th Jul 2007, 14:22
Check where on the airfield the elevation is specified. A 50 foot variation between one end of an 800m runway and the other is equates to a gradient of less than 1:50 - difficult to see by eye. If you are checking the altimeter "on chocks", and there is such a slope, it could account for the error.

OC619

ericferret
16th Jul 2007, 18:46
Have you tried tapping the guage!!!!!!!!!!!!

Large aircraft have built in altimeter vibrators. Small aircraft rely on engine vibration to help remove internal friction. FAR 43 Appendix E allows a swing of up to 75 feet at 0-1000 ft due to internal friction..

So you need to ensure that the guage has been "vibrated" before checking the error.

Captain Punishment
16th Jul 2007, 19:22
slim slag, where do I find the document from which you quoted:

"I have a reference for 'in the order of' 75ft. AIM 7.2.3.a.3"

Do you have a web link at all please?


erricferret, the alt was check at threshold elevation with engine running so would have plenty of vibration after power checks etc.

slim_slag
16th Jul 2007, 19:52
http://www.faa.gov/airports_airtraffic/air_traffic/publications/ATpubs/AIM/Chap7/aim0702.html

or

http://tinyurl.com/25xl4g

7.2.3.a.3

which is just over half way down

IO540
16th Jul 2007, 22:12
I think the original poster is talking about a G-reg.

The bi-annual pitot check referred to above is done on N-reg only. This can cost up to 500 depending on how far one is away from the nearest firm that has the special equipment for doing it. A lot of people use a firm in Cranfield which charges best part of 200 for driving down. Mind you, flying to Cranfield isn't that much cheaper nowadays ;)

On a G-reg this check isn't done - presumably because the UK doesn't have any significantly elevated airfields so nobody really cares about altimeter errors.

slim_slag
16th Jul 2007, 23:59
So why did you quote FAR 43, which didn't even answer the original poster's question? One suspects my answer was of more use :)

fireflybob
17th Jul 2007, 00:20
Check where on the airfield the elevation is specified. A 50 foot variation between one end of an 800m runway and the other is equates to a gradient of less than 1:50 - difficult to see by eye. If you are checking the altimeter "on chocks", and there is such a slope, it could account for the error.


In the UK the published aerodrome elevation is the highest point on the manoeuvring area. As I said in my previous post above the altimeter check should be done on QNH (not QFE) with a knowledge of the exact elevation amsl of the ground on which the a/c is situated.

(Note - in some states, Greece for example I think, the aerodrome elevation is the height amsl of the aerodrome reference point - typically the centre of the main runway.

Captain Punishment
17th Jul 2007, 19:12
Yes chaps the aircraft in question is G registered so FAA rules would not really apply.

Not found anything in my POM as this was my first port of call.

Everyone that I have spoken to seems to suggest 50ft is acceptable error, but still do not have conclusive documentation to support our theory to add weight to my dispute with my group member.

ericferret
17th Jul 2007, 19:16
The current UK position is for a full pitot static leak check and an altimeter calibration every year.

CAA LAMS A 1999 ISSUE 2 section 8 pages 13 and 16

slim_slag
17th Jul 2007, 19:22
Yes chaps the aircraft in question is G registered so FAA rules would not really applyThe AIM isn't a rule book. It's really a book of good advice, of which quite a lot could be considered relevant to non US ops, so it would apply (unless you don't like taking advice :) )

Of course if you are flying on an FAA certificate, you fail to follow the advice and have an accident, the FAA could come after you for being 'careless and reckless'. But for your purposes the advice about altimeters should be considered applicable. IMO. Altimeters work the same way all over the world, and the one you are using was probably made and certified in the USA anyway.

But as I asked earlier, if anybody has a reference to 50ft then that would be interesting too. Nobody has as yet delivered. Until they do, and it's in a CAA document, then I suggest that the AIM reference I provided you is the best ammunition you have got.

High Wing Drifter
17th Jul 2007, 22:37
I agree with Fireflybob's -75/+50, that is what I was taught have used consistently with five examiners (PPL, IMC, CPL and IR + renewals) across three schools (maybe they weren't listening!). Fireflybob's explanation makes perfect sense. The US system doesn't use millibars and 0.01 in/hg is about 9', I'm not sure if that is relevant to this discussion.

Anyway, section 10.2.1 here provides the information on reporting pressure: http://www.caa.co.uk/docs/33/CAP746.PDF

FWIW, I just tried to find the allowable limit in a Seneca PIM, I know everybody uses -75'/+50' (assuming aircraft specific +/-50'), but I can't find any mention of it.

Arclite01
17th Jul 2007, 22:48
I would suggest:

1. Get them recalibrated anyway for your peace of mind - one or other could be out of limits, sometimes there are issues around the static source and just venting them to the cockpit static can solve it, sometimes 2 different altimeters (different makes) can have a variation too !)

2. Buy new ones if you are unhappy it'll cost your syndicate 300 GBP for 2 of them (not a lot really)

3. Leave your syndicate if you can't resolve the issue diplomatically - just think if you are having hassle over a small issue like this - what are you going to do when you have a major replacement issue like an engine or propellor combo ?

Arc

IO540
18th Jul 2007, 06:24
Certified altimeters cost a lot more than that - more like 1500 each.

slim_slag
18th Jul 2007, 10:18
Most flight manuals on PA38/PA28 type aircraft list the max pressure error correction as plus or minus 50 ft.Both the PA-28-161 (REPORT: VB-1188) and PA-28-181 (REPORT: VB-790) say:

The altimeter error is less than 50ft unless otherwise placarded

Cannot find anything similar in the few Cessna manuals I have to hand and don't have any grumman manuals. Not sure it helps anyway, all that says is that in a piper, if it's out by more than 50ft you just stick a placard next to the altimeter and you are good to go.

Well, this is all very interesting. The only regulation I can find which comes close to this subject are the ones which say your Mode C needs to be within 125ft of what the altimeter reads, and that's only for IFR. It seems to me that there is no rule which says the altimeter needs to be accurate if under VFR, it just needs to be there. So unless somebody can come up with something I think the OP is pretty much out of luck, unless he uses the AIM, which appears to be a dirty word :). One learns something every day, it appears these +/-50ft and +50/-75ft 'rules' are without any regulatory basis. They are just pieces of advice handed from person to person - unless somebody can demonstrate otherwise.

High Wing Drifter
18th Jul 2007, 12:51
There is an RVSM requirement for +/-80'. Tacitly, there is the PEC fo 50' added to Decision Altitude for CAT 1 precision approaches. That must have a basis in the +/- 50' - especially when one considers that non-precision approaches have the PEC included in the Min Decision Alt.

But other than that I agree, it is down to the FM/PIM/POH and an understanding of other influencing factors.

slim_slag
18th Jul 2007, 12:52
Just reread the bit in the PA-28-181 manual which talks about altimeter error being less than 50ft. It doesn't mean what it was claimed to mean, and I originally missed that too and I have slapped my wrist.

What this actually refers to is when the alternate static source is being used. As we all should know, when alternate is on, the altimeter uses cabin air for it's static pressure, not the air outside.

What the manual actually says is, that when alternate static is on, the altimeter error should be less than 50ft. i.e less than 50ft difference to the altitude indicated when using outside air.

So, totally irrelevant to the subject.

I think..... :)

DFC
18th Jul 2007, 12:54
The reference for the pre-flight altimeter check is ICAO Document 8168 Volume 1 Part 3

For altimeters which read from 0 to 30,000ft it is +/- 60ft and for those that read 0 to 50,000ft it is +/- 80ft.

However, one must use a certified altimeter check location to get an accurate indication on an apron or use the runway threshold.

If flying from a small unlicensed GA airfield such as for example Popham, one has to make allowances for such things as;

1. Aerodrome elevation survey accuracy (if any survey was done as none is required).

2. Slope of aerodrome surfaces. The aerodrome published elevation will be the highest part of the landing area.

3. The calibration of the instrument used to measure the QFE and how the operator obtains the QHN from that. It is not simply a case of sticking an old altimeter on the desk in the clubhouse and setting it to the field elevation before reading QNH. Ask a certified met observer how they obtain QNH at their airfield for the correct method. The QHN at a nearby aerodrome also introduces errors.

All of the above could easily mean that a serviceable altimeter could indicate more than the +/- 60ft pre-flight from such a place.

Note that the tollerences quoted above apply below 3000ft elevation. DOC 8168 provides a table for checking altimeters above this height.

Regards,

DFC

slim_slag
18th Jul 2007, 13:12
Well there you go, DFC is correct. None of us got +-60ft, the correct answer. Just goes to show that some of these safety based 'rules' we all bandy around are nothing more than old wives tales.

Captain Punishment
18th Jul 2007, 15:49
errice ferret, do you have a weblink for this at all, as I do not hav ethe book?

slim slag, the quote from you is AIM is good advice, unless you don't like taking advise.... This is exactly my problem, I know what the advice is, but the guy running the group sees this as advice and relys on his own unqualified judgement, so advice is no good to aid my argument.

I will need hard fact / UK regulation to change this guys mind!

If it were my choice I would have alt checked and calibrated probably if anymore than 20 - 30 ft out on a regular occasion.

The quest goes on for a documented 50ft figure....

Who will bring it to the table...?

fireflybob
18th Jul 2007, 15:57
Just reread the bit in the PA-28-181 manual which talks about altimeter error being less than 50ft. It doesn't mean what it was claimed to mean, and I originally missed that too and I have slapped my wrist.

What this actually refers to is when the alternate static source is being used. As we all should know, when alternate is on, the altimeter uses cabin air for it's static pressure, not the air outside.

What the manual actually says is, that when alternate static is on, the altimeter error should be less than 50ft. i.e less than 50ft difference to the altitude indicated when using outside air.

So, totally irrelevant to the subject.


slim slag, well the CAA appendix to the PA28-161 Flight Manual I am looking at says the PEC does not exceed 50 ft in all flight phases - no reference whatsoever to use of alternate static source!

slim_slag
18th Jul 2007, 16:55
This is what I am using..
http://i106.photobucket.com/albums/m258/slim_slag/proon/Warrior0002.jpg

Though I think DFC has already solved the problem.

fireflybob
18th Jul 2007, 17:05
slimslag, I am not disputing your reference with respect to alternate static but what I am saying is that there is also a max PEC of +/- 50 ft for "normal" operation - I will scan the reference next time I am down the airfield and post it!

DFC ref to the ICAO Doc is quite correct but I would suggest that if the AFM declares a more accurate tolerance this would override ICAO!

slim_slag
18th Jul 2007, 17:13
I don't think you are disputing anything, I am posting the page as there is so much gubbins posted on here and I think it's important to reference one's sources. You cited a CAA appendix which I don't have, I am more than happy for you to be correct. The only thing that matters is the truth be out :) Referencing authoritative sources makes a higher quality debate, IMO, got touched on in the thread for reasons people don't post here anymore.....

ericferret
18th Jul 2007, 19:10
www.faa.gov/aircraft/

open regulations and policies
open federal aviation regulations

look for FAR 43 appendices

DFC
18th Jul 2007, 20:06
The position error correction is a factor but it is part of the total system error.

When considering the siting of the static vent, the designer will find it very hard to find a place where there is absolutely perfect static air pressure.

What you see indicated on the altimeter may include an instrument error i.e. put the altimeter on the test bench and it may be out by say 30ft.

It may also include an error due to the position of the static port or in the case of obtaining the static from the alternate source inside the aircraft. The manufacturer is only as far as I remember required to tabulate the PEC in the flight manual if it is more than 50ft. The PEC may vary with mode of flight, power setting etc etc bu as per the flight manual will not be any more than 50ft.

If one takes the PEC as the maximum 50ft then there can only be 10ft of instrument error in the same direction to take the indication out of the ICAO limit. Of course if the PEC and instrument errors are opposite then they may cancel each other out.

The requirement from the pilot's point of view are simple. Pre-flight when at the altimeter check location and with an official QNH set ensure that the altitude indicated is within 60 or 80ft as appropriate of the datum. If it is then go flying if not call an engineer.

Regards,

DFC

PS remember that PEC also affects the ASI.

slim_slag
18th Jul 2007, 20:44
If one takes the PEC as the maximum 50ft then there can only be 10ft of instrument error in the same direction to take the indication out of the ICAO limitFAR 43 Appendix E allows a 20ft tolerance at a pressure altitude <= 1000ft. 20 + 50 = 70. Would that mean a properly working altimeter with an acceptable PEC cannot be used to go flying if you use the ICAO reg?

DFC
18th Jul 2007, 21:15
slim,

Think of it - If the aircraft is at rest and the power is off on a calm day, what PEC error could there be?

Regards,

DFC

slim_slag
18th Jul 2007, 21:46
Ah yes, never thought of that!

They say there is no such thing as a stupid question but I reckon I just stretched it a bit.....

DFC
18th Jul 2007, 22:01
Not stupid at all.

When I re read my first post, I can see where I confused you.

PEC applies to the aircraft in motion or perhaps if the airflow from the propeller passes over the static port AKA Cessna 172. One would hop that propeller flow would not use up all the 50ft available.

Regards,

DFC

slim_slag
18th Jul 2007, 22:41
Nah, lot simpler than that, just a brain f@<hidden> on my side.

slim_slag
19th Jul 2007, 10:13
Well, all very interesting. Being one of those who has used +/-50ft because that's what my instructor said and I didn't ask why, then I told that to students, who didn't ask why. No doubt some of those are instructors telling their students +/-50ft and they aren't asking why. And so on and so on.

So we are handing down what is essentially rubbish. It's simple questions like the first which gets one to think about this.

Anyway. Two more questions.

1) If ICAO says +/-60ft. Why does the AIM advise +/- 75ft?

2) If ICAO says +/-60ft at 1000ft, why does FAR 43 says the altimeter has to be +/-20ft at 1000ft?

DFC
19th Jul 2007, 17:13
The AIM applies in the US only.It is not an international standard and the FAA have many differences from ICAO.
FAR 43 Appendix E refers to the bench testing of an altimeter at a repair station. Such tests are done with the altimeter in a sealed chamber fed with ISA air and gently vibrated. The value your quote is just one part of the test and ther are others such as the leak check and hysteresis tests have tollerences also.
Unless the air at the entry to the altimeter in the aircraft matches the test chamber it is unlikely that the exact same tollerences will be acheived.
Regards,
DFC

ericferret
19th Jul 2007, 19:54
I looked at CAAIPS and found that the altimeter section has been deleted.
I then dug out my old copy of CAIPS and to my surprise found that the tolerance was not quoted.

The CAA refer to manufacturers recommendations and ICAO standards.
I would suggest that for UK registered aircraft the 60 ft limit is correct.
Although as most altimeters are manufactured in the USA the FAR 43 might also be correct!!!

However LAMS does say calibration insitu is acceptable.
Therefore FAR 43 does not apply.

50 foot sounds good to me, nice round figure on the safe side.
What engineers would do in the old days before we were all converted into part replacement specialists and thinking was still regarded as being relevant to the job.

Where is the CAA desert when you really need it!!!!!!!!!!!!!!!!!!!