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Author Topic: Altitude - QFE or QNH  (Read 15909 times)

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tarbat

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Altitude - QFE or QNH
« on: June 26, 2008, 01:10:10 PM »
A question on the altitude readout on the aircraft list (and map).

Firstly, I've got RB setup to get the QNH pressure from my local airport (EGPE).  And in preferences, I've got the transition altitude set to 3000ft.  So what does this actually do?

Am I correct in thinking that above 3000ft, altitude displayed will be taken directly from what the aircraft sends (ie based on 29.92in)?  Or is then adjusted in some way by RB using the current QNH pressure?

And below 3000ft, is the altitude calculated using the QNH pressure to give the altitude above sea-level?

If my interpretation is correct, why do I often see RAF Tornados flying at 10ft AMSL, when I can see that visually they're at about 150ft AMSL?
« Last Edit: June 26, 2008, 01:13:47 PM by tarbat »

AirNav Support

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Re: Altitude - QFE or QNH
« Reply #1 on: June 26, 2008, 01:20:32 PM »
Thats correct above the transitional altitude it won't adjust the altitude reading from the aircraft. Below it will use the local QNH to get the reading.

How far are the RAF Tornados away from EGPE, as there could be pressure difference between the airport and where they are which causes the alititude to be out.

 
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tarbat

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Re: Altitude - QFE or QNH
« Reply #2 on: June 26, 2008, 01:30:15 PM »
The RAF Tornados are typically about 20 miles from EGPE (Inverness airport).  They operate out of Lossiemouth, and then approach the bombing range which is about 4miles from my home.

Do military aircraft set their altimeters using civilian or military procedures (ie do they use 29.92, QNH, or QFE)?
« Last Edit: June 26, 2008, 01:52:21 PM by tarbat »

AirNav Support

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Re: Altitude - QFE or QNH
« Reply #3 on: June 26, 2008, 01:41:14 PM »
We don't know I am afraid. Anyone with military background know?
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Allocator

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Re: Altitude - QFE or QNH
« Reply #4 on: June 26, 2008, 01:58:45 PM »
Tarbat,

If you are talking about aircraft using the range, when the aircraft check in on the range frequency, what pressure are they given by the Range Controller - I'd guess QFE.

If the aircraft are in transit, then they will be using the RPS (Regional Pressure Setting) which is the lowest forecast sea level pressure for the next hour.  For example, if its 1505 hrs, the RPS will be valid from 1500 to 1600 hrs.

Of course, aircraft flying at 150ft amsl will be using the Radar Altimeter.  When you say you can see them at 150ft amsl, you do mean actually over the sea, not the land?

So, how does this answer your question?  I'm afraid it doesn't really, but you might be able to work out something from this!  It is possible that the aircraft are flying on the QFE which gives height above the highest part of the range, then they could in theory descend below "ground level" when over the sea.

dudbaker

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Re: Altitude - QFE or QNH
« Reply #5 on: June 26, 2008, 02:11:11 PM »
Hi

This is my educated guess.  Below trasition level they would use reginal pressure settin or QNH.  Approaching an airfield then QFE.  On a bomb run to a range then radio altimeter.  The height has be be very acurate for bomb release.  They are lobbing this bomb by pulling up just befre release so they do not get caught in their own blast.  They might not drop a bomb but send a tone at release point and the range then caculates where the bomb would have landed.  Hoping to be with in 60 feet.  Also, they have terrain following radar to control height.
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tarbat

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Re: Altitude - QFE or QNH
« Reply #6 on: June 26, 2008, 02:16:31 PM »
Thanks Allocator, I'd hoped you might know!!

Currently the controllers are giving a QFE 1003, and current QNH from Lossie, Kinloss, and Inverness are all 1004, so I guess that might explain it.  The RAF aircraft have their equipment set to a QFE, whereas RB works on the basis of the aircraft having the equipment set to 29.92 (I'm guessing), or does it assume the aircraft is set to QNH pressure?

When I see the aircraft, they're over the sea, although the range is only about 10ft AMSL - it's the bit of land just to the west of my Home location in this screenshot:

« Last Edit: June 26, 2008, 02:53:02 PM by tarbat »

tarbat

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Re: Altitude - QFE or QNH
« Reply #7 on: July 25, 2010, 07:03:56 PM »
So, does the altitude transmitted by an aircraft reflect a pressure setting of 1013.2, or the local QFE?  More importantly, what assumption does Radarbox make about the altitude transmitted - does RB assume the altitude reflect a pressure setting of 1013,2, or QFE?

And do fast military jets (like the Tonka) follow the same principles when transmitting their altitude?

EDIT : Sorry about the missing picture in previous post - I had to remove a lot of images from Flickr because of copyright infringement of hi-res outlines.
« Last Edit: July 25, 2010, 07:27:02 PM by tarbat »

dudbaker

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Re: Altitude - QFE or QNH
« Reply #8 on: July 26, 2010, 08:39:23 AM »
Hi

Just monitored the last 3 landings at STN.  Altitudes were 320, 340 and 370.  The airfield is 348.  Airfield QNH 1016.  This implies airfield QNH is used.  Working on 1 millibar is 30 feet.  If 1013.2 were used then aircraft would land at circa 260. 

While checking QNH on ATIS I note aircraft are being asked to switch on Transponder on start up.

Dudley
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Valiant, Victor, Shackelton, Canberra, VC10, Tornado Typhoon 737 747 A320 A300 Engineer.

Chris11

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Re: Altitude - QFE or QNH
« Reply #9 on: July 26, 2010, 08:42:20 AM »
Mode C transponders transmit based on 1023. and I am pretty sure Mode S will be that same. The radar and computer system of the ATC will automatically convert from 1013.2 to the local QNH when the aircraft passes the transition level of the day.

From the previous post it appears as if ANRB does the same.

DaveReid

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Re: Altitude - QFE or QNH
« Reply #10 on: July 26, 2010, 08:58:48 AM »
Mode C transponders transmit based on 1013. and I am pretty sure Mode S will be that same.

Yes, no debate about that.

Quote
The radar and computer system of the ATC will automatically convert from 1013.2 to the local QNH when the aircraft passes the transition level of the day.

From the previous post it appears as if ANRB does the same.

Indeed so - provided that you have an airport for which a METAR is available selected in your SmartView, and the "Auto Set QNH" box ticked (or else a QNH set manually in Preferences), then RadarBox will do the adjustment for you.
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