Testing altimeter and ASI

[djpacro]

I'll stick with the use of the word "normal" in this context.GPS is a very good method for obtaining PECs for homebuilt aircraft.

CASA's AC:

 

A GPS Method can produce accurate results over the entire speed range of the aircraft and is relatively simple and cost effective to carry out.

EAA members may read their excellent articles on this subject wrt homebuilt aircraft. The method is also acceptable for FAR 23 production aircraft - refer the FAR 23 Flight Test Guide. Sure it has some limitations but .... off topic.

[Dafydd Llewellyn]

Dafydd, surely mounting or trailing any such things would be in breach of whatever regulations apply to modification of an LSA ?

Well done that man! Yes indeed it's a modification; this is why it's totally inappropriate to talk about this in the context of the two-yearly instrument calibrations; it's not for calibrating the instruments - it's for calibrating the pitot-static system. However in certification testing, when this IS appropriate, one is always working on an experimental certificate under CASR 21.191(a) or 191(b).

 

CASA's AC: EAA members may read their excellent articles on this subject wrt homebuilt aircraft. The method is also acceptable for FAR 23 production aircraft - refer the FAR 23 Flight Test Guide. Sure it has some limitations but .... off topic.

I agree, it's an acceptable method - just not one I would choose to use. However, it can't be used for speeds for which it is necessary to dive the aircraft - so it won't cover the full speed range of most aircraft. But these methods are NOT for the two-yearly instrument calibrations, for which they are entirely inappropriate.

 

 

[old pilot]

I just read in the latest Sport Pilot the rules on biannual testing of ASI, altimeter, and leakage in pitot lines. It was news to me. Is it news to others? Anyway, I can test the ASI against my GPS in two directions and record that in the maintenance log, the other tests I will need to talk to my LAME. It is strange that these tests have not been suggested to me before and don't appear in my maintenance manual. Thanks to the temporary Tech man for writing the note.

Hi these tests have been in the tech manual for some time now

Old pilot

 

 

[bilby54]

Anybody who depends on the ASI to fly a plane is living dangerously.

Would you mind explaining that in more detail as I would have thought it to about the most critical instrument for normal operations

 

 

[Teckair]

Would you mind explaining that in more detail as I would have thought it to about the most critical instrument for normal operations

If you fly long enough you will find yourself in an aircraft with a non working airspeed indicator, there can be a number of causes for this, pitot tube blocked, cover left on etc. There is a direct relationship between throttle setting and nose attitude and that is how you have to fly without an ASI. If you cannot do this then when the ASI stops working you will be in danger.

 

 

[Dafydd Llewellyn]

That's a common misconception held by people who normally fly aircraft that have a healthy stick-free stability (stick force versus speed gradient - i.e. you have to push harder to fly faster, and pull harder to fly slower, than the trimmed speed). The stick force versus speed gradient for, say, a Lancair 320, is virtually nil; and it's also pretty small for most aircraft in landing mode - normal symptom is that they will not hold a trimmed airspeed in the landing configuration - and it can become negative (unstable) in a baulked-landing situation. In any of those situations, the ASI is vital.

 

Yes, one can get an aircraft with a U/S ASI (most common cause - mud wasps) and most of us have had this experience - around a circuit and back on the ground by attitude & power - but it's vastly easier to do if it has normal stick-free stability characteristics.

 

I would go further, to say that it's a common misconception held by pilots that because all the aeroplanes they have experienced fly pretty much the same, that's an inherent property of anything with wings. The reason all certificated aeroplanes fly pretty much the same (if you ignore the effects of sheer mass) is that somebody sweated blood to make certain they did. It's far from inherent, and some homebuilts that have never passed a certification process, are very different to this.

 

 

[facthunter]

Any one who flew much from West Maitland would have encountered the wasp mud blockage of the pitot. You should actually pick it up during the take-off run ( partial blockages produce weird results) but IF you don't, the circuit must be flown without it. IF it's suss it must be ignored. You can't use something that is half right or unreliable in any way.

 

Setting cruise power for a known configuration and weight will produce something quite close to a predictable airspeed result if you are flying at a level that is constant, but applying the thinking to a powered approach that is not flying a fixed glideslope ILS or VASIS, into a known wind starts to get hairy. The usual thing is to err on the high side with your estimate of the airspeed, but you should be wary of letting the groundspeed (IF the winds are significant) unduly affect you feeling of airspeed judgement as it will especially if your plane has little evidence of wind noise. An open cockpit like a Tiger Moth has the noise of the wires and control feel to help, but some aircraft provide little indication from these sources. Aileron response is a good indicator on a conventional plane you are familiar with..Nev

 

 

[sain]

For those interested in how to build and use their own manometer to conduct ASI / pitot / static testing the following link may be of use:

 

http://www.iflyez.com/manometer.shtml

 

 

[bilby54]

I have done several instrument checks on aircraft and really it takes very little effort to check the sense lines during a 100 hrly. I have found that problems due to faulty instruments show up long before any testing is done - water in the pitot can give wild speed indications as well.

 

The Tulsa instruments are rugged and almost bomb proof but can only be overhauled by an approved instrument shop whereas the RAA type are throw away items especially if the aircraft spends its life on grass runways. I agree that the manometer is the best way to calibrate, or at least, check instruments but there is no harm in doing a speed reference check in flight with the GPS

 

 

[JG3]

the manometer is the best way to calibrate, or at least, check instruments but there is no harm in doing a speed reference check in flight with the GPS

 

The manometer only calibrates the instrument. There are far more errors that come from the installation, mostly from the static source. What we really need is the real airspeed, not just the indicated. To get this is easy these days. Just pick a calm morning and do a four-way GPS run. No way a single run or even a back-track run is adequate, must do the 4-way. Hold a constant indicated airspeed, and fly N-W-S-E legs, each long enough to see both indicated and ground speed stabilized. Sum the four GPS readings and divide by four. If there's more than 10 kts difference between any legs then there's a wind up there. The averaging should compensate for this bias, and it does pretty well, but the less wind the better.

 

JG

 

 

[Dafydd Llewellyn]

the manometer is the best way to calibrate, or at least, check instruments but there is no harm in doing a speed reference check in flight with the GPS

The manometer only calibrates the instrument. There are far more errors that come from the installation, mostly from the static source. What we really need is the real airspeed, not just the indicated. To get this is easy these days. Just pick a calm morning and do a four-way GPS run. No way a single run or even a back-track run is adequate, must do the 4-way. Hold a constant indicated airspeed, and fly N-W-S-E legs, each long enough to see both indicated and ground speed stabilized. Sum the four GPS readings and divide by four. If there's more than 10 kts difference between any legs then there's a wind up there. The averaging should compensate for this bias, and it does pretty well, but the less wind the better.

 

JG

 

Does your aircraft have a Flight Manual (Pilot's Operating Handbook)? If so, look in the "performance" section - you should find the airspeed system error ("Position error") data there. If it is there in the FM/POH, why do you need to determine it again? It's an inherent result of the location of the pitot and static sources on the aircraft, and thus, not subject to change, unless you change the shape of the aircraft in an area that affects those sources. The instruments themselves, however, are subject to change over time - so checking them is a periodic requirement. You do NOT need to fly the aircraft to do this.

 

 

[Dafydd Llewellyn]

the manometer is the best way to calibrate, or at least, check instruments but there is no harm in doing a speed reference check in flight with the GPS

The manometer only calibrates the instrument. There are far more errors that come from the installation, mostly from the static source. What we really need is the real airspeed, not just the indicated. To get this is easy these days. Just pick a calm morning and do a four-way GPS run. No way a single run or even a back-track run is adequate, must do the 4-way. Hold a constant indicated airspeed, and fly N-W-S-E legs, each long enough to see both indicated and ground speed stabilized. Sum the four GPS readings and divide by four. If there's more than 10 kts difference between any legs then there's a wind up there. The averaging should compensate for this bias, and it does pretty well, but the less wind the better.

 

JG

 

Y'know, this reminds me of the dog that chases cars: he hasn't thought it through - what would he do with a car if he caught one? Neither have you.

 

So, you go out there and fly a GPS course, and average this & that and come up with a total airspeed error - for one particular speed. That error comprises the total of the instrument error plus the position error plus the true airspeed correction for density altitude. So you need a calibrated ASI, a calibrated altimeter, and a calibrated OAT (we'll ignore the humidity correction and the Mach No. correction). Let's assume you had those calibrated instruments, so you could make the various corrections and calculations, and thus come up with the position error at that indicated airspeed.

 

It will almost certainly be slightly different to what it says in the Flight manual. What are you going to do about this? Amend the Position Error data in the flight manual? Guess again, pal, that's illegal unless you're an appropriately authorised person. In any case, every airspeed in the flight manual is given as IAS, assuming the FM position error and zero instrument error - so you'd have to amend every airspeed that is quoted in the FM.

 

 

[Guest ozzie]

so technically every one is grounded again? except 95:10

 

who does these instrument checks and sign offs?

 

and add another incidental cost to affordable aviation.

 

 

[facthunter]

What's the point if they are not TSO'd instruments in the first place?. How would they be legal and how likely would they be to calibrate properly? I didn't think TSO'd instruments were required up till now (IF they are). If an ASI is consistent and not far out it won't be unsafe if you check the unaccelerated stall speed. Mostly poorly located static sources are responsible Nev

 

 

[Dafydd Llewellyn]

so technically every one is grounded again? except 95:10who does these instrument checks and sign offs?

and add another incidental cost to affordable aviation.

No, of course not. Just use a manometer (you can make it yourself) to calibrate the ASI and also to check the plumbing for leakage; and send your altimeter to an instrument shop at the required intervals. Forget the position error, it's a given in any certificated aircraft. The instruments do not need to be TSOed; they need to be within the limits of CAO 108.56 when you check them, that's all. There's nothing new about any of this.

 

 

[Guest Crezzi]

Without a transponder check per AD/RAD/47 in the last 2 years your aircraft is not permitted in controlled airspace ( including class E).

Transponder not required for Class D

 

Cheers

 

John

 

 

[frank marriott]

so technically every one is grounded again? except 95:10who does these instrument checks and sign offs?

and add another incidental cost to affordable aviation.

No not everyone - I have mine checked by an AAME, have done so since new - [it is an extra expense but as I use Class C airspace - necessary] - shop around because different places have largely different prices for the same job.

 

 

[Guest Andys@coffs]

Dafydd

 

Their point is valid imho. Within RAAus Aircraft I would guess that 75% or more use the Cheap non TSO'd altimeters. No LAME instrument fitter is going to issue a serviceability tag for these generally Chinese sourced instruments because there is no manual that defines the checks, the tolerances and the approved repair methods that are to be used if they need to be calibrated, nor at what point calibration via adjustment is actually needed. (Is the instrument out, or just sh!tty?....or more like reality its Sh!tty and out)

 

As such what determines whether a particular instrument is serviceable?

 

Andy

 

 

[M61A1]

Some cut and past from the tech manual- starting with the intro, then moving to the relevant section-

 

SECTION 4.2.4

 

PERIODIC INSPECTIONS

 

INTRODUCTION

 

Aircraft operated under the auspices of CAO 95.10, 95.32 and 95.55 shall be maintained

 

in accordance with the manufacturer's maintenance manual. Where no manufacturer's

 

manual exists the aircraft should be maintained to the schedules contained in this

 

Manual. Aircraft types used for hire or reward shall always be maintained in accordance

 

with the manufacturer’s schedules.

 

The following Notes and Maintenance Schedules are generalised for use on aircraft of

 

all types of construction. Cross out items not applying to your aircraft and insert in the

 

spaces provided additional items suitable for checking in your aircraft.

 

When maintenance is performed a log-book entry must be made. This entry must

 

include the signature of the person authenticating the maintenance, their name (in block

 

letters), the date the entry was made and RA-Aus membership number.

 

And then on to-

 

INSTRUMENTATION

 

17. Altimeters on aircraft used to enter controlled airspace are to be checked every 12

 

months against a currently certified altimeter or appropriate test equipment for the

 

task. Altimeters are not to deviate by +/- 100 feet up to the limit of the instrument or

 

10’000 feet whichever is the lower.

 

18. Altimeters on aircraft not used to enter controlled airspace are to be checked every

 

2 years against a currently certified altimeter or appropriate test equipment for the

 

task. Altimeters are not to deviate by +/- 100 feet up to the limit of the instrument or

 

10’000 feet whichever is the lower.

 

19. Air speed indicators are to be checked every 2 years against a manometer or

 

against a GPS using test runs in opposite directions. Airspeed indications shall not

 

vary by +/- 4kts. Aircraft with more than one ASI shall not have variations between

 

the instrument readouts by more than +/- 2kts.

 

20. Pitot and static systems are to be checked for leaks every 2 years using a device

 

capable of holding and indicating pressure for a minimum of 2 minutes without loss of

 

pressure.

 

Nb. Remove tubing from instruments before pressurising the system to avoid damage

 

to the instruments.

 

21. All transponders used in aircraft entering controlled air space are to be checked

 

against CASA AD/RAD/47. Compliance with the inspection is to be noted in the aircraft

 

log book.

 

 

[Dafydd Llewellyn]

DafyddTheir point is valid imho. Within RAAus Aircraft I would guess that 75% or more use the Cheap non TSO'd altimeters. No LAME instrument fitter is going to issue a serviceability tag for these generally Chinese sourced instruments because there is no manual that defines the checks, the tolerances and the approved repair methods that are to be used if they need to be calibrated, nor at what point calibration via adjustment is actually needed. (Is the instrument out, or just sh!tty?....or more like reality its Sh!tty and out)

 

As such what determines whether a particular instrument is serviceable?

 

Andy

If it meets the error limits given in CAO 108.56, it's servicable, I think you'll find. If you have one of the Cheep altimeters, you may learn that it was never within a Bull's roar - but that's the reason for the calibration requirement. Check CAO108.56 against the RAA TM excerpt above - I think you will find the TM is a verbatim quote of CAO 108.56 (as, really, it must be, if we are to maintain the same separation standards as the rest of the airspace users).

 

 

[M61A1]

I have a box of instruments collected over the years, most them the cheap type. I took them to work and asked the avionics guys to put them of the "suck & blow" machine, which they did as some training for some apprentices. The end result, 4 out of 5 (alt & asi) did quite reasonably (passed) , the one that failed wasn't by much. I was pleasantly surprised

 

 

[440032]

Ah, Dafydd, CAO 108.56 was cancelled 9 July 2013.

 

 

[Dafydd Llewellyn]

Ah, Dafydd, CAO 108.56 was cancelled 9 July 2013.

Yes, the CAO s are being slowly phased out. What has it been replaced by, do you know?

 

 

[facthunter]

If you can understand the rules we will fix it so you can't Signed CASA ...We are not happy till you are not happy. Nev

 

 

[djpacro]

Option A - read the earlier posts in this thread for the current rules

 

Option B - don't read earlier posts and carry on