998/988 millibars
 

I know this is an old one, but I really scared myself last Thursday...

QFE at home base was 988. I was being radar vectored onto the ILS in VMC as part of an IMC refresher lesson. I was told to set QFE 988 and descend to height 1800ft, report level.

On reaching and reporting I was at 1800ft the controller said, 'Confirm height 1800ft QFE 988 millibars.' I checked and confirmed it. My instructor then pointed out politely that I was in fact on 998 millibars, so I was 300ft lower than I thought.

What scared me here was not so much that I made a mistake - it's an old one and I should have seen it coming - but that the controller queried my altitude (presumably based on the mode C readout) and I looked and saw what I expected to see, not what was actually there. It's certainly taught me to check more carefully!

Tim

Slightly OT...
 

Hi,

Talking about mode C. Where does this altitude info come from? If it's the main altimeter then it would report the same error (due to the incorrect subscale setting).

I probably should know the answer, but I don't think it's ever been explained to me. :uhoh:

Cheers,
DB.

It comes of the altitude encoder which is connected to the static source but nothing to do with your altimeter. It transmits your alt based on 1013mb which the controller then converts to QNH.

Yes the altitude comes from the altimeter, but I'm not sure if the reference is from what is selected on the subscale though.

Edited: Disregard - I just see that Bose-X has replied - so it's encoded from a pressure of 1013 mbar!

There are three answers to this.
Firstly, some transponders have their own encoder built in (Filser for example).
Secondly, a "blind encoder" can be used and can be thought of as an external add-on box for the transponder which is hidden away and has no controls.
Lastly you can have an encoding altimeter. This looks like a normal altimeter but has a seperate (electronic) output for the transponder.
In all cases they are connected into the static system and transmit the height relative to 1013. The controller can input the QNH into the radar system which can then convert your transponder output to Altitude.
If you had been at edinburgh or Glasgow yesterday you could have made an even bigger error as the QNH went down to 966mb which would give an error of around 1400ft!

I much prefer instrument approaches on QNH - I then don't have an altimeter change at a relatively high workload time, have time to cross check against my GPS altitude and ATC has time to say 'confirm level' when I am nowhere near the ground. I do however have to remember numbers other than 200, 600, 800 for my MDA/DH!

<quote> It transmits your alt based on 1013mb </quote>
Thanks for the clarification. That actually sound familiar... perhaps I was explained that at one time! :ugh:

This is one reason why i could never understand why all altimeter subscales don't appear as the odometer on cars.
I have only ever come across one altimeter like so, ok a bit more expensive but less likely to be set wrong.

It happens to the best of pilots, which is why NATS have recently been issuing a notam, via the daily nav warning briefing, warning when low pressure in certain areas of the UK is expected and everyone should pay heed to this.
Something worth bearing in mind, where ever you may get your briefing from..

Best to avoid QFE altogether and stay on QNH, like the rest of the world does.

When IFR it is handy to know the altitude of the airport you are going to. Gross errors of this sort can often be avoided by comparing the QFE of your base airfield with your destination. In your case presumably if you had done that the chnage would not have made sense.

Except Military of course :O

Is it only the UK's armed forces (or even just the RAF) who routinely use QFE? What do the worlds other armed forces use? I would have thought that naval aviators use QFE, but what of the others?

There are plenty of civvy airfields that use QFE:

Ronaldsway
Liverpool
Newtonards
Dublin
Weston
......blah blah blah

In general I think it is 'The UK establishment' that uses QFE. Its use elsewhere is limited.

In terms of armed services, I believe there was a thread about a year ago that concluded that QFE's use by armed forces outside the UK was limited. A lot of country's forces routinely operate into fields that you couldn't practically use QFE for.

Any airfield will give you QFE but that doesn't mean you have to use it. You just read it back and ignore it, and stay on QNH.

Those that have instrument approaches should give you the option of flying the IAP on QFE or QNH. Commercial instrument pilots fly on QNH only, AFAIK.

I'm quite aware that in the UK QFE is used extensively in civvie land. My question about the military was in response to XL319's response of "except the military" in response to IO540's statement "use QNH as the rest of the world does".

Perhaps the question would have been better phrased as: is it only the UK military (or only the RAF) that routinely use QFE, or do other countries armed forces also use QFE?

But looking back, thats exactly what I did ask... :ugh:

Edited to say:
Sorry mm_flynn, for some reason I didn't see your reply when I originally posted this. What types of airfields is QFE of less use for? Hot and/or high?

I did see this, but still c@cked up...

My home base is of course a military airfield.

I can see the logic for the military. Essentially all military IAPs are the same - at least all PAR/SRA/ILS ones are, to a great extent. When they set up a temporary base at e.g. Baghdad, they can use the same procedures and they are always the same - 1800ft QFE to intercept the glideslope at 6 miles, for example, height at 2 miles = 600ft...

Something to be said for familiarity when operating in that sort of environment.

Tim

Depends on the company. The company I work for insist on using QFE and it make sense to me. Somehow it is comforting to know that you won't crash until the altimeter reads zero (assuming of course the little window has the right number in it). But each to their own.

<Pedant mode on>
Newtownards: The QHN and QFE are the same as it's about 5 feet above sea level :}

<Pedant mode off>

But your point is taken.

Cheers,
R.