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Acceptable IAS discrepancy ?

10 Posts

Hey there !

Looked into the TB20 PIM, this forum and the www, but did not find any usable information.

What would be, on our GA plane, an “acceptable” IAS discrepancy between main and stby indicators ?

when crosschecking on takeoff at 50 IAS, i usually have 3 to 5 knots discrepancy between those instruments.

LFBZ LFBP, France

5kts certainly makes a difference. In a single it is the difference between a greaser and a thump, in an MEP it is potentially the difference between life and death (flying close to the red line).

I was once asked to fly an MEP which was behaving very oddly. We did some GS triangles and determined that the IAS was out by about 9kts. I said that in my opinion the aircraft was not airworthy. Had we had an EFATO, real or practised, before we realised there was a problem, we might have died.

OK, it’s not quite such a big issue in an SEP, but why would you not get them checked and calibrated?

EGKB Biggin Hill

I have never seen a TB20 with two IAS indicators… what is the second one? Is there a second pitot tube, is it heated, etc? If it is heated, is there a second CB for it? If not heated, that could be your answer

Administrator
Shoreham EGKA, United Kingdom

TB20 G500
Same as this one, but with a GNS530 + 430 installation.
https://www.euroga.org/forums/g500-gtn650-gtn750-in-a-tb20gt/4614?page=2

Not my aircraft, so can’t get it checked.

Main indicator is G500
Second is standby, just on the left of it.

I don’t have the aircraft PIM at disposal, but looked through the ENAC documentation I have (ENAC is operating the same aircraft with same installation), and there is no description of the anemometric chain.
Can’t find online the G500 supplemental for the PIM.

There is only one pitot, so I guess there is only one source.

Last Edited by jeff64 at 14 Aug 14:00
LFBZ LFBP, France

I would suggest doing the three-headings TAS test, mentioned a few times e.g. here, to establish which of the two is right.

I suspect the original ASI will be right…

Maybe someone familiar with the G500 can post whether there is a calibration option on it. @garryiae or @wigglyamp may know.

Administrator
Shoreham EGKA, United Kingdom
23.1323 Airspeed indicating system.
(a) Each airspeed indicating instrument must be calibrated to indicate true airspeed (at sea level with a standard atmosphere) with a minimum practicable instrument calibration error when the corresponding pitot and static pressures are applied.

(b) Each airspeed system must be calibrated in flight to determine the system error. The system error, including position error, but excluding the airspeed indicator instrument calibration error, may not exceed three percent of the calibrated airspeed or five knots, whichever is greater, throughout the following speed ranges:

(1) 1.3 VS1 to VMO/MMO or VNE, whichever is appropriate with flaps retracted.

(2) 1.3 VS1 to VFE with flaps extended.
KUZA

These are design specs, and the highlighted text limits the difference between IAS and CAS for a perfectly working indicator – what does apply in maintenance?

Last Edited by Cobalt at 14 Aug 19:24
Biggin Hill

For gliders in the BGA CAMO system, we use + or – 2 kts when checking ASI at every annual.

Nympsfield, United Kingdom

Peter wrote:

I suspect the original ASI will be right…

Maybe someone familiar with the G500 can post whether there is a calibration option on it

There is a field calibration procedure for the GDC74A which is the air data computer used with the G500 and G1000. However, it’s only for re-calibrating the altitude if it fails the FAR 91 tests (Garmin document 190-00303-83 Rev D).

From personal experience with G500s, I would almost guarantee that any error will be in the original mechanical ASI that is retained as the standby instrument. Best check is to perform a full in-situ calibration of both instruments using a currently calibrated high resolution (probably digital ) air data test set and see what transpires.

Avionics geek.
Fairoaks. EGTF

Xtophe wrote:

For gliders in the BGA CAMO system, we use + or – 2 kts when checking ASI at every annual.

For the Garmin G500 system, the STC instructions for continuing airworthiness provide the following test requirements:

Avionics geek.
Fairoaks. EGTF
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