Light aircraft static altimeter max permisible tolerance on ground?
Guest
Posts: n/a
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.
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.
Join Date: Jan 2003
Location: Norfolk
Posts: 1,057
Likes: 0
Received 0 Likes
on
0 Posts
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
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
Join Date: Jan 2001
Location: He's on the limb to nowhere
Posts: 1,981
Likes: 0
Received 0 Likes
on
0 Posts
Most flight manuals on PA38/PA28 type aircraft list the max pressure error correction as plus or minus 50 ft.
The altimeter error is less than 50ft unless otherwise placarded
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.
Guest
Posts: n/a
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.
But other than that I agree, it is down to the FM/PIM/POH and an understanding of other influencing factors.
Join Date: Jan 2001
Location: He's on the limb to nowhere
Posts: 1,981
Likes: 0
Received 0 Likes
on
0 Posts
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.....
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.....
Join Date: Mar 2002
Location: Euroland
Posts: 2,814
Likes: 0
Received 0 Likes
on
0 Posts
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
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
Join Date: Jan 2001
Location: He's on the limb to nowhere
Posts: 1,981
Likes: 0
Received 0 Likes
on
0 Posts
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.
Thread Starter
Join Date: Jun 2007
Location: UK
Posts: 44
Likes: 0
Received 0 Likes
on
0 Posts
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...?
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...?
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.
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.
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!
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!
Join Date: Jan 2001
Location: He's on the limb to nowhere
Posts: 1,981
Likes: 0
Received 0 Likes
on
0 Posts
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.....
Referencing authoritative sources makes a higher quality debate, IMO, got touched on in the thread for reasons people don't post here anymore.....
Join Date: Mar 2002
Location: Euroland
Posts: 2,814
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jan 2001
Location: He's on the limb to nowhere
Posts: 1,981
Likes: 0
Received 0 Likes
on
0 Posts
FAR 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?
Join Date: Mar 2002
Location: Euroland
Posts: 2,814
Likes: 0
Received 0 Likes
on
0 Posts
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
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