Transponder reading too high

[Ringway Flyer]

We have transisted the M/cr low level route twice today. Out bound, we were advised by M/cr ATC that we were above the 1300' limit. Both altimeters, set to the correct QNH showed that we were at least 200' below 1300'. I set one altimeter to 1013, and the transponder readout was about 200' higher.

On the return we transisted at 1000', and still had a problem, setting off the alarms when I went up to 1100'.

I have spoken to the duty manager, and he was very nice about it, and not unduly concerned - just don't do it again!

We have been using that route for the past 20 odd years and are well aware of the height limitation.

So - do transponders need re-calibrating? Any advice would be appreciated. I don't know the model number, but it's a Garmin mode S.

[Wodrick]

I would be more tempted to go for the encoding altimeter, that is the variable, the transponder only churns out what it is fed.

I have no experience of your equipment but am a 40 year Licensed Radio man (Rtd)

[LASJayhawk]

Is this a helo or small fixed wing? Many use a remote encoder, that has a high and low altitude adjustment. Even in a/c with an encoding altimeter they can get wacked out of calibration. Oddly enough the old mechanical IDC remote encoders are dang near bullet-proof, the newer solid state electronic ones are less reliable.

In the US we test them every 2 years (FAR 91.411 and 91.413)

You need an avionics person to do a correlation check on the aircraft.

[Wodrick]

I've never been involved with anything smaller than a 737 so guessing really :)

[Ringway Flyer]

Thanks for your comments - confirming that they do need looking at sometimes. We have now located an engineer locally who is to carry out a re-calibration.
RF

[Gas Bags]

What type are you jockeying? Did the problem exist with a transponder switch?

[NutLoose]

As both altimeters read correctly to each other, it will probably be the encoder which on light stuff is separate.. It is fed by a separate tube off the altimeter circuit, can be adjusted, though they are known to fail.

It is also checked biannually in the UK too..


Chances are you will have a

Product Information Page for Garmin GAE 43 Altitude Encoder (http://www.seaerospace.com/garmin/gae43.htm)

Or

Amer-King (http://www.ameri-king.com/altitude_encoder.html)

:O

[jxk]

It is also checked biannually in the UK too..Biennially?

[NutLoose]

Oops finger trouble and spell checker, of course i meant every two years

I forgot to add this link, please see

http://ad.easa.europa.eu/blob/easa_ad_2006_0265_CN.pdf/AD_2006-0265-CN_1


..

[AvChappy]

Sounds like your Altitude Encoder is grumpy mate, here are some common types I've seen fitted to the smaller toys;

Amer-King (http://www.ameri-king.com/altitude_encoder.html) (AK 350)

Products | ACK Technologies, Inc. (http://www.ackavionics.com/products.htm) (ACK A-30)

The ACK A-30's have given me the least trouble and tuning them is a breeze :ok:. You may also have a Static system problem, but that seems unlikely from what you've mentioned.

[latetonite]

I do not see what your altimeter setting has anything to do with what is encoded.

[NutLoose]

They are normally plumbed on the same line, the fact his Alts were at the Alt shown but the encoder was showing on the transponder as 200ft higher points to the encoder as the problem, the Alts simply giving a reference to work off.

[bazmcneil]

as per AD Rad 43/47, I suggest you get hold of an IFR 6000 transponder test set, pump up the P/S & see what is happening. Probaly a leaking static system. If it is leaking, split the P/S system in half and analise that way.
Wombat, avionics guru.

[bazmcneil]

Just posted, but after thinking about it, doubt your equipment faulty( Transponder, encoder) you have a static leak most likely