Incorrect altitude shown on Radar

[David Torkington 867509]

Hi all,

 

Whilst flying on multiplayer in the IFDG A318, I have a 1000ft discrepancy between my indicated altitude and that which is shown on ATC's radar. This showed up last night whilst in a holding pattern. My indicated altitude was fl100, yet fl110 was showing on radar.

 

As usual, I always 'calibrate' the altimeter with the local QNH by pressing 'B', although I understand there may be slight altitude differences between pilot and ATC due to software. But 1000ft is a little too much

 

One thing I suspect [and I hope someone here has faced the same issue] is that the Lee Hetherington TCAS gauge I installed in the Ken Mitchell panel is somehow affecting the altitude. The reason I say this is because I noticed that last night whilst trying to find a symptom for the 1000ft error, fl110 was in fact displayed in the TCAS window.

 

For your information, I'm using FS2004 and SB3. I tried to replicate the problem 'offline' with different QNH's / altitudes so I could post a screenshot link, but the TCAS and altitude display were the same. I don't know whether this issue applies to other aircraft I have installed.

 

Any suggestions? By the way, I love the IFDG model with Ken Mitchells panel, so swapping to another type isn't an option... Cheers

[Lennart Vedin]

My indicated altitude was fl100...As usual, I always 'calibrate' the altimeter with the local QNH by pressing 'B'

When you say FL100, it mean you should NOT use local altimeter setting.

 

Fligth Level are used at and above "Transition Level (TL)"=18000ft in U.S. , while there are different in Europe. In Europe at and above local Transition Level typical 5000 to 7000ft you should use standard altimeter setting 1013 hPa and not local setting.

 

ATC will [Mod - Happy Thoughts]ign you if use local or standard altimeter setting by say "Flight Level XXX" (use standard altimeter setting), or say "XXXX feet" (use local altimeter setting).

 

ATC should as well say local QNH pressure before takeoff and at first decent under Transition Level. ATC may not need to say local QNH pressure if you already have information by ATIS (indicate by pilot say e.g. "...with information BRAVO..." at first contact).

 

Second:

You should not calibrate by pressing 'B' at all. Because your current local QNH at your position with the information your FS have at the moment may difference with the local QNH that is the current airport information. I.e. the local pressure at you position it not the one to use. ATC [Mod - Happy Thoughts]ign a common QNH you and all aircraft should use, and ATC decide when to change QNH based on the pressure at the airport. The only way you can know QNH to use, is the information you get from ATC or confirmed ATIS.

[David Torkington 867509]

Thanks for the quick reply Lennart, it seems I've got into a lazy habit of following the FS instructions rather than following good procedure - many thanks for the excellent information, short and to the point! I can get the METAR info no problem from various sources and ATC always give this data.

 

So I have another small but significant problem, because I cannot adjust the QNH using the Ken Mitchell panel I have installed [unless someone can correct me]. Do I have to change the panel completely, or is there a gauge I can add to resolve this?

 

Also, would the difference in 'local' QNH [which from memory last night was 0987Mb] to QNH1013 result in a height discrepancy of 1000ft? I'm not disputing the proper procedure, I'm just trying to search for any other reason for such a large difference between ATC radar and my indicated f/level.

 

Best regards

 

Edit :- Ok, I've corrected myself by eventually finding the QNH adjustment in the Ken Mitchell panel by hovering all over the altitude gauge [there must be a manual for the panel somewhere I haven't read] and the difference in altitude is indeed 'significant'...

 

Thanks again

[Lennart Vedin]

standard pressure at sea-level is 1013 hPa. at 18000 ft the pressure is 1/2 i.e. 500 hPa.

 

Approximative:

so 18000 ft difference is 500 hPa difference. you did have 26 hPa difference from standard (1013-987=26).

 

26 hPa difference is 26/500*18000 ft = 936 ft. Yes your setting 'cause approx 1000 ft fault.

[David Torkington 867509]

Exactly right Lennart, I must admit I checked some conversion calculations after your previous post and found an error of approximately 900ft due to my mistake. However, I was too embarr[Mod - Happy Thoughts]ed to confess... Anyhow, if ever there are ATC students being mentored, then I'm the one to test them out - if only for their patience!

 

Best regards,

 

Dave.