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Transponder altimetry

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Transponder altimetry

Old 6th Jun 2019, 07:12
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Transponder altimetry

I am sure this has been discussed before, I have searched but have been unsuccesful!

Question:- If transiting VFR beneath hanging airspace on a given QNH, a pilot and all around him (or her) will be using the same datum for height reference - their altimeter.
Controllers however will be using data transmitted from the aircraft's XPNDR, (mode S) which we know to be more accurate and will almost certainly be transmitting a figure that is unlikely to be the same as the altimeter. So there will be two definitions of the vertical dimension. the altimeter and the XPNDR. (Pilot's and controllers if you will)

The controlller decides that there's been a vertical incursion, but the pilot maintains that his altimeter was correctly set and accurately flown - Discuss.

This is a hypothetical question I'm not asking for a mate! :-)


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Old 6th Jun 2019, 09:11
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Your transponder is not necessarily more accurate, but will transmit based on standard pressure setting i.e. 1013 hPa. Radar is likely to convert this to show altitudes below transition altitude, but their pressure setting may not necessarily be the same as yours. Normally the difference between the main airport QNH set on the radar and your remote QNH are small, but on a very windy day a long way from main airport the differences can be larger (still unlikely to be more than 100 feet difference). Sometimes the QNH set on the radar can be wrong, but in the UK these days that is unlikely with an automatic update.
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Old 6th Jun 2019, 09:57
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If it's a TMA you're referring to, you should be using the QNH of an airfield beneath that TMA.
You're allowed a +/- 200ft error if there is a claim you've infringed.
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Old 6th Jun 2019, 09:59
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Originally Posted by Dan Dare View Post
Your transponder is not necessarily more accurate, but will transmit based on standard pressure setting i.e. 1013 hPa. Radar is likely to convert this.....
and therein lies the gem that my logic was missing - thank you.

So the XPNDR (cockpit) indication is based upon 1013 and that reading is converted (to that of the local QNH) before being displayed on the controllers radar screen....

Thanks again



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Old 6th Jun 2019, 10:01
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As Dan Dare said.
In my neck of the woods, the radar system corrects the data sent by the altitude encoder for local QNH, as per the QNH areas promulgated.
Above transition level, the encoder should be correct. In my experience, it's usually accurate to about a hundred feet.

The only likely source of inaccuracy would be if there was a significant variation of QNH from one side of a promulgated QNH area to the other side (as could happen with severe weather, or a frontal passage) and the pilot had set the altimeter to read airfield elevation prior to departure, and had not received the (significantly different) area QNH.
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Old 6th Jun 2019, 10:02
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Originally Posted by chevvron View Post
If it's a TMA you're referring to, you should be using the QNH of an airfield beneath that TMA.
Yep I understand that - wasn't aware of the +/- 200ft. Thanks.

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Old 6th Jun 2019, 12:37
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IIRC, mode S is capable of (and is usually equipped to) report level in 25ft increments whilst mode C reports are in 100ft increments.
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Old 6th Jun 2019, 12:44
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Originally Posted by Tarq57 View Post
The only likely source of inaccuracy would be if there was a significant variation of QNH from one side of a promulgated QNH area to the other side (as could happen with severe weather, or a frontal passage) and the pilot had set the altimeter to read airfield elevation prior to departure, and had not received the (significantly different) area QNH.
Not sure what inaccuracy you are thinking about. As far as radar processing and display is concerned, the altitude displayed will be based on the appropriate QNH that has been put into the display system. This may be different to that shown on the aircraft altimeter, but this is not uncommon in the circumstances originally described.
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Old 6th Jun 2019, 17:32
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Originally Posted by LookingForAJob View Post
IIRC, mode S is capable of (and is usually equipped to) report level in 25ft increments whilst mode C reports are in 100ft increments.
My understanding too
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Old 10th Jun 2019, 19:19
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Originally Posted by LookingForAJob View Post
Not sure what inaccuracy you are thinking about. As far as radar processing and display is concerned, the altitude displayed will be based on the appropriate QNH that has been put into the display system. This may be different to that shown on the aircraft altimeter, but this is not uncommon in the circumstances originally described.
Simply that the "spot" qnh at the departure aerodrome might be significantly different to the area qnh, in changing weather ie, a frontal passage.
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Old 10th Jun 2019, 22:28
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I want you (because my book likes it this way ) to switch your C Mode Off in case of discrepancy .
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Old 11th Jun 2019, 05:48
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Originally Posted by Dai Whirlybird View Post
Yep I understand that - wasn't aware of the +/- 200ft. Thanks.
Dai your txpdr output should indeed have a +/- 200 foot tolerance applied to it.Unfortunately current CAA/NATS policy is only to to give that tolerance to verified squawks.Those wearing conspicuity squawks do not get the tolerance.Problems are occurring where transiting under airspace veils the over reading txpdr has triggered CAIT (controlled airspace infringement tool).A lot of the froth about infringements is being generated by this problem.I liken it to getting a speeding ticket for for doing 29 in a 30 zone.Unforunately the simple sums involved seem beyond current regulators.My information comes from current ATCOs who are also practising Commercial Pilots they cannot believe the problems being created.Effectively yet more of our class G airspace has been stolen.My advice at present is to plan to fly at least 400 feet vertically below controlled airspace if squawking an unverified code to allow for transponder tolerance and the inevitable turbulence etc.Hopefully AOPA seem to be finally waking up to what is going on in the real world .Regards Stampe

Last edited by Stampe; 11th Jun 2019 at 05:52. Reason: error
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Old 11th Jun 2019, 06:04
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It's not always possible to fly as much as 400' below the lower limit of airspace. For example, the situation where the base corrresponds to MSA, such as below a significant amount of the Class G airspace under the London TMA.
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Old 11th Jun 2019, 06:21
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Originally Posted by ShyTorque View Post
It's not always possible to fly as much as 400' below the lower limit of airspace. For example, the situation where the base corrresponds to MSA, such as below a significant amount of the Class G airspace under the London TMA.
ST I totally agree for example the procedures into and out of Redhill are virtually unworkable for an aircraft seeking to avoid a CAIT event.The Stansted TMZs are now avoid zones unless willing to operate at very low level.The safety of operations in the south east has been hugely diminished for transponding GA aircraft.Unfortunately the result of current NATS/CAA policy is that very unwisely a lot of transponders are being switched off.This surely is not what NATS desire.Restore the transponder tolerance to all squawks and the situation becomes workable,manageable and fair for all parties.A significant number of the supposed infringements will disappear.Regards Stampe

Last edited by Stampe; 11th Jun 2019 at 06:25. Reason: addition
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Old 11th Jun 2019, 08:38
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Originally Posted by Stampe View Post

Unfortunately current CAA/NATS policy is only to to give that tolerance to verified squawks.Those wearing conspicuity squawks do not get the tolerance.
Got a link or reference to this policy? Iíve been out of the NATS game for a short while but the +/-200í was permitted for all aircraft.




Having taken avoiding action more times than I care to remember and dealt with airspace infringers many times over the years - both on the R/T and via telephone after the fact - all I can say is if youíre flying near controlled airspace then *talk to an ATCO* or, at the minimum, listen out and wear a listening out squawk. No ATCO is stealing airspace from you. We are merely protecting aircraft flying inside a controlled environment - usually along with 100s of people inside - along with our licenses.

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Old 11th Jun 2019, 09:31
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Packer, good advice. However, at busy times in UK (e.g. a typical good weather weekend) it is increasingly difficult to get a call in to ATC, to get a service of any kind in Class G.

The simple fact is that the more controlled airspace there is, the more people need to fly through it, or get close to it.

It has got to the point in some areas of UK where it is pointless even trying. One good example is Farnborough Radar, who can get "swamped" with callers. The poor standard of R/T of some pilots exacerbates a difficult situation - sometimes ATC have to carry out a "questions and answers" session, to the detriment of everyone on the frequency. Having one controller looking after three areas of responsibility makes it even worse.
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Old 11th Jun 2019, 20:31
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Base of CAS is 2500ft, ILS traffic is at 3000ft, you're trundling along at 2200ft underneath CAS but your transponder overreads by 200ft. Suddenly you hit turbulence and climb 200ft. Your transponder is now showing 2600ft (within tolerance, perfectly legal). ILS traffic now gets a TCAS RA. That's the kicker.

TCAS will sometimes - not often, but occasionally - set off RAs even on traffic transponding at 2400ft outside CAS, even when both appear straight and level.

Pilots and airlines do not like TCAS RAs one bit, especially in the approach phase.
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Old 11th Jun 2019, 21:03
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Originally Posted by Stampe
Unfortunately current CAA/NATS policy is only to to give that tolerance to verified squawks.Those wearing conspicuity squawks do not get the tolerance.

Got a link or reference to this policy? I’ve been out of the NATS game for a short while but the +/-200’ was permitted for all aircraft.
Back in my day verified mode C could only be associated with a validated mode A target. A target on a conspicuity code was not considered to be validated even if identified by some non-secondary means, hence unknown traffic or that on a conspicuity squawk had to be avoided by 3000ft or 5000ft (depending on circumstances).

As for the +/-200ft thing, that only applied to assigned levels (and consequently only to identified targets).

I know things have changed but I thought some of these basic principles still applied; with some, at least, in SERA. Or meringue?
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