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Testing altimeter and ASI


pmccarthy
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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.

 

 

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I don't know how accurate ASI is when compared to GPS speeds. If you are flying directly up and down wind, then you should get an accurate reading, but when there is a cross wind I cannot see that you will get the required accuracy.

 

 

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It is very difficult to use a GPS to calibrate your ASI to the required accuracy. It requires you to fly a triangular course and then compute the crosswind effect to arrive at the actual airspeed. You also have to factor in density height, and of course fly very accurately. A few minutes with Google will dig up various spreadsheets that have been made up for the purpose. OK for basic accuracy tests.

 

Much easier is to build a manometer and do it in the comfort of the hangar. I have found this to be very simple and accurate, and can also be used for static instrument tests and pitot and static pressure tests. Again, use Google for all the details.

 

The requirement has always been there, along with the transponder checks. Now that we are flying above 5000 feet AMSL and mixing with RPT and IFR traffic it becomes even more important.

 

 

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I believe they are more interested in the altimeter as that is critical in keeping you out of the way of other planes I wonder how many planes are flying without accurate height indications

and I to wonder about that too,,,,makes it a worry when someone gives an ALT in a position report and it could be wrong by a few hundred feet. In the Yarra Valley with two big airfields, a few private strips, helo ops at vineyards and meat bombers inbetween, tootleing around with a dodgy ALT because you didn't read the rules could kill people. I've stood on the ground and watched pilots waffle through the YCEM circuit and wondered if they're suicidal or just got the wrong altitude, I've seen a few near collisions from this and there has been a midair in the past in the circuit here!

This is our biggest problem in RAA, we think the rules don't apply to us and then when big brother comes along and says well actually apart from a few exemptions the rules DO apply we all cry foul!

 

I heard an older pilot once say the Reg's are written" in the blood of pilots past", sounds serious doesn't it.

 

Matty

 

 

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Might be worthwhile to check how many people understand "altimetry" around the traps. Apart from having an altimeter that is not too accurate, the setting of the subscale to current area QNH is probably not done. If the actual height of the departing aerodrome is set carefully it is accurate locally for a while depending on whether some significant weather is moving through the place. I wouldn't suggest using QFE. It's not standard procedure and may ingrain bad habits of associating ground level with zero on the altimeter as well as ignoring ALL other heights that should be adhered to. Nev

 

 

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bi·an·nu·aladjective \(ˌ)bī-ˈan-yə(-wə)l\

: happening twice a year

 

: happening every two years

 

Always gets me? the above taken from a dictionary

 

spacesailor

The correct term is "bienniel" meaning an event lasting 2 years or occuring every 2 years. Its most common use is in the plant kingdom where biennial plants bloom in their second year & then die. The dictionary you are using has an incorrect second meaning.

 

 

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GPS method normally used to get PE corrections (so needs a calibrated ASI beforehand). A CASA AC refers.There is a CAO about these regular calibrations of altimeters and ASIs.

No, not normally - the "normal" method of getting PE corrections is by the use of a trailing-cone static plus a pivoting-vane pitot. This measures the calibrated airspeed directly (if connected to a freshly calibrated ASI instrument. ) Any method that tries to measure airspeed by measuring groundspeed, is less accurate; and in general, less likely to give reliable results. If one makes a video recording of the "ship's" ASI and the "calibration system" ASI simultaneously, it is possible to get the whole story on PE corrections, for all flap positions, in a few minutes.

 

Anyway, I suspect the TM is confusing the instrument error - which can be checked with a simple manometer - with the ASI system error (position erroe) which is an inherent property of the location of the pitot and static sources, and is not subject to change over time.

 

See CASA AC 21.40 for methods for determining the position error - which is a normal part of type certification; and CAO 108.56 for routine re-calibration requirements for instruments. The two should not be confused.

 

 

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As a L2/LAME I have sometimes observed a lack of appreciation for the responsibilities that come with owning an aircraft. The owner is considered to be the Maintenance Controller which means that they are responsible for understanding and ensuring all aspects of the RAAus Technical Manual, CASA regs, manufacturers maintenance schedules, AD's, SB's, etc are complied with. Your L2/LAME can help point you in the right direction, however the aircraft owner is the person ultimately responsible. Heaven forbid, should a failure to comply with any rules/regs be found to contribute to a fatal accident the lawyers would have a field day at your expense. Moral of the story is that aircraft owners need to do their homework by reading and understanding all the documents that relate to the ongoing serviceability and maintenance of their aircraft. A range of solutions are availabe for checking calibration of instruments, but for transponders it must be checked by a CASA Radio LAME using pretty expensive calibrated test equipment. Without a transponder check per AD/RAD/47 in the last 2 years your aircraft is not permitted in controlled airspace ( including class E). I offer this as food for thought for anyone who owns an aircraft, that you may better appreciate the responsibilities that are yours.

 

 

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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.

I think I'd say, an adequate method, for homebuilts, if applied carefully. Mainly of value because it requires no more apparatus than a GPS, outside air temp, altimeter, and ASI (means you have to calibrate three instruments instead of two). We're usually after higher accuracy, in certification testing. With the GPS method, you really need to fly a "wind triangle" for each speed, which makes it a tedious business to get more than about half a dozen points on the curve for each configuration; and it requires very accurate flying. Using video (or digital) recording of data from a good calibration setup, one can rapidly get sufficient data to refine the result to around 0.5 knot accuracy, and to see what is going on close to the stall, where the PE often changes rapidly; and it does not require very accurate flying. Any method has its particular peculiarities; you have to get them right, to obtain good data. I've built a number of pivoting-vane pitot setups, and the trailing-cone static is a straight Woolworths exercise, and the result, if done properly, is as close to an "absolute" reference as one can get. The AC is 21.40

 

 

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I don't know how accurate ASI is when compared to GPS speeds. If you are flying directly up and down wind, then you should get an accurate reading, but when there is a cross wind I cannot see that you will get the required accuracy.

You will never get an accurate airspeed reading with a GPS, ground speed and air speed are two completely different things.

 

 

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Better watch out for a blooming Bienniel check then. (This is another delayed post )

 

Re the GPS. It will give you are very accurate groundspeed If you have a very accurate figure for the wind you get an airspeed from that data. combined... Flying a triangular course should allow you to derive that (somewhat tediously).

 

For all practical purposes why wouldn't an ASI calibrated accurately on fine pressure readings suffice PEC's and static problems are constants and the indicated speed you have for stall speed (actual stall for a given condition say AUW) doesn't change in practice Using the pressure inside the cockpit as a static source, makes things look good. ie SLOW stall and fast cruise..Nev

 

 

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Third line. Nev

Is this the Third line you are referring to?

 

Flying a triangular course should allow you to derive that (somewhat tediously).

If so that certainly will not give accurate wind details.

 

 

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An altimeter should be much easier to check than an ASI and more important. An ASI should only be used as a rough guide and unless there is something obviously wrong with it it probably should be left alone. Anybody who depends on the ASI to fly a plane is living dangerously.

 

 

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Firstly, the start of this thread was to do with the TMs comments on how to perform the mandatory periodic calibration of the flight instruments. That is covered by CAO 108.56; go look it up. The key word is INSTRUMENTS.

 

However the TMs comments mentioned - I consider, inappropriately - a technique that can (tediously, and not very accurately) be used to determine the error inherent in the AIRCRAFT's pitot-static system - usually called the "position error", because it is the result of the position of the pitot and static sources on the aircraft. Because the pitot and static sources do not normally walk around on the aircraft, I do not see any necessity to re-determine the position error, unless the pitot or static sources have been affected by a modification.

 

AS a quite separate subject, when one is flight-testing an aircraft, it is a prime priority of any sane test pilot to calibrate the ASI system as just about the first step after ensuring that the engine will not overheat in the course of the testing - because no sane test pilot wants to unwittingly exceed the design structural airspeed limits. That applies equally to a homebuilt or a new type certification prototype. If it's a TC prototype, the design standard normally sets an upper limit on the maximum position error. Also, there is an upper limit of the static source error, because that affects the ability to use the altimeter to stay outside controlled airspace. For these purposes, the most useful tools, in my experience, are:

 

(i) A trailing-cone static - essentially a large - about 300 mm diameter - plastic funnel with lots of 50 mm holes cut in the sides, to act as a drogue, at the end of a length of plastic tube so it trails a couple of metres more than the wingspan, behind the tail - with a set of static pressure holes in the wall of the tube, about 2 metres ahead of the funnel. The tube is blocked between the static holes and the funnel. This costs about $10 or so to make, and it will give an accurate static source provided the altitude is held constant and the speed is steady. It can be compared with the ship's static by switching an altimeter from one to the other. There are limits on the permissible altimeter error cause by the ship's static source.

 

(ii) A pivoting-vane pitot - essentiall a small weather-vane carried on a boom about one wing chord ahead of the wing, well clear of the slipstream, with a pitot intake at its front end, that is held by the vanes so it always points straight into the airflow. Provided the pitot always points straight into wind, it can be as simple as a square-cut piece of thin-wall tube.

 

The combination can be connected to a calibrated ASI which will then read the correct indicated airspeed, allowing for the instrument error, again provided the aircraft is maintaining height at a steady airspeed.

 

When one gets to the upper end of the aircraft's speed range, it is generally necessary to dive somewhat - and then errors can occur with the above sort of calibration setup, due to the lag in the change in static pressure; this can be offset to a degree by "dynamic lag calibration", whish can be done using a 44-gallon drum and the brake-booster vacuum source on a handy car. However, that's another story.

 

 

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