Altimeter Setting

[Flyco]

We have no laid down procedure for setting altimeters prior to flight. I normally set mine to QFE, so giving zero height at take-off. However, many other countries, including the US, use QNH, thus giving runway height above sea level at take off. The RAF standard is QFE for local flying but QNH is also widely used, and away from the circuit a ‘Regional QNH’ is used below 3000 ft amsl.

One problem with QFE is that for airfields above 3000 ft (I think), the QFE value cannot be set on the altimeter. Another possible problem which has recently been raised in the DCS Forum is that, allegedly, for the A-10C, setting other than airfield QNH will cause small errors in the IFFC. I find this difficult to believe, but it could be true, and it might explain the consistent minor errors that several of us have experienced in delivering GBU-12s.

I suggest therefore, that we adopt the procedure of setting QNH at start up and using it for take-off. If everyone is agreeable, we should write this into our SOPs and use it for all aircraft. There is nothing to stop QFE being used for local circuit work etc, provided that the take-off is made on QNH. There is no difficulty with using QNH, it is the most common procedure used worldwide.

I will certainly start using QNH routinely in future, only setting QFE during recovery.

[Neil Willis]

The errors attributed to altimeter settings would certainly explain the random seeming inaccuracies for some weapons.

Although I think it goes against what we are led to believe is the doctrine used in the RAF (You can probably enlighten us in that respect Alan?), I am happy for us to run with QNH and see what effect that has.

I suggest we test the theory that it is affecting weapons delivery, and take it from there?

[Flyco]

Obviously I am 20years out of date about RAF procedures, but I suspect they have not changed significantly.

For operational flying, the decision to set QFE or QNH would be dependent on what the sortie was. I suspect that the norm was to take off with QFE, largely, I suspect, for convenience - flap retraction heights etc. Certainly in any emergency requiring a return to land it would simplify matters.

In flying training, QFE would most normally be used, but it was never a rigid rule, and would again depend on what the sortie was.

In practice then, there was no hard and fast rule. If you were remaining in the circuit or the instrument pattern QFE would be used, but if clearing the circuit there would be no reason not to set QNH for take off. QFE would almost invariably be set for an instrument or a visual recovery to a military airfield, and decision heights would be quoted for QFE. On the other hand any involvement of civilian ATC would involve use of QNH, although QFE could be used at the pilot’s request. I seem to remember that within NATO, both QFE and QNH were in general use.

I will,certainly use QNH from here on, particularly when flying the A-10C, although I would probably use QFE when recovering and landing. Contrary ain’t I.

On balance, particularly in view of the heights of some of the airfields in use in Nevada and the Caucasus, I would recommend using QNH for take off and departure, but see no problem with setting QFE for recovery. The only final question is what should we use as the Transistion Altitude - the height at which we change from QNH to SPS during the climb out. There are good arguments for setting it above the height of the highest ground in the area/country. Hence In the UK it was 3000 ft, in the USA it was 18,000ft, and I do recall flying in countries where it was some intermediate height say 5000or 8000ft. I confess that ‘internally’, particularly flying the Hog, I assume 18000 ft, and that would,be my recommendation for us.

So I would recommend:
Takeoff with QNH, unless you are remaining in the circuit, when you may use QFE if you prefer.
Set SPS (29.92) passing the Transition Altitude of 18000 ft.
Reset QNH when descending below the Transition Level for recovery (but QFE is acceptable)
Set QFE when you call ‘Inbound’ and use that for landing.

Technically when flying on SPS, you are flying at a ‘level’ not a ‘height’, and the level at which you change from SPS to QNH (or QFE) during descent is termed the Transition Level. In addition the Transition Level will change depending on the atmospheric pressure, and would be passed to you by ATC. However, I don’t think it is worth going to those extremes, especially since DCS ATC doesn’t pass Transition Levels. (Finally, to complete the picture for the pedants, there is a volume of airspace between the Transition Level and the Transition Height, known as the Transition Layer, in which it is impossible to fly - it will always be less that 500 ft thick)

I’ll bet you wished that you had never asked!

[Neil Willis]

It all makes sense to me.

I have always worked on 6,000 ft as the transition altitude on the Caucasus map, and as you said, it is 18,000 ft on the Nevada map under FAA regulations. Can you clarify how you set QNH. I presume it is to set the airfield altitude on the altimeter, thus giving mean seal level as zero feet?

It'd be good to have general outline as well as specifics for the SOPs, and as you've got the handle on that, I am happy for you to go ahead and draft a set of procedures to be inserted there.

[Flyco]

Normally, when you first contact ATC for start clearance they will give you runway in use, wind, OAT, QFE and QNH. DCS is a bit mean in that respect.

As you surmised, to get the QNH, simply adjust the altimeter to the actual airfield Altitude, and the sub-scale gives the QNH (I make a note then on my knee pad - yes, I do have a pad with pencil holders strapped to my knee -even in the sim). To be exact, you should set it at the runway threshold, rather than on the dispersal pan, to allow for airfield that slope -you can have a different QNH for each runway.

There is always a bit of ‘stiction’ in altimeters, so traditionally pilots always tap it to get it get the needle to zero, despite the fact that modern ones are electrically driven with small vibrators inside them - I confess that I still do sometimes myself tapping it!

I will draw up a very brief set of procedures for using the altimeters and when to set what.

[Raiden5706]

My thoughts on this is simple:

QFE take off and landing

QNH operating below our prescribed SPS altitude

SPS operating above prescribed altitude.

[Flyco]

Raiden, that is pretty much the RAF system for UK, although if the intention is to leave the circuit, it is common to set QNH from take off.

However, it has been asserted that the A-10C systems make use the QNH altitude on take off, with the result that you will get a small error entered into the IFFCC, which is enough to give you a small error in some target deliveries. I don't know how true this is, but we have observed a consistent small impact error, using GBU-12 on a number of sorties - exactly what the Forum claim says will happen.

For that reason, I am going to use QNH for take-off. If I find that the weapon error is still present I will, reconsider.

Formally, once outside the circuit area, and before reaching the Transition Altitude, you should change to QNH. If the TA is only 3000 ft, as in UK, this is a minor consideration, but in the US the TA is 18000 ft, which means that for 15000 ft you are climbing with a meaningless altimeter setting. We are not sure what the local TA is In the Caucasus, but I would be surprised if it were not more than 3000ft.

Whatever the case, we have been discussing revising SOPs (Standard Operating Procedures), and my view is that we should adopt the procedure I outlined. The SOPs will be circulated for comment and amendment before being finalised, but that is my view for the present. Once adopted, we would hope that everyone complies with them.

[Neil Willis]

I'm with Alan on this, What we need primarily is accurate delivery of weapons. If using QNH achieves that, then we must alter our procedures to give us the required reliability of weapons.

It should override a relatively minor detail of procedures if the end goal is achieved without compromising safety in the vicinity of the airbases we use.

That of course assumes that we'll see an improvement in weapons delivery as a result. If we don't, we can revert back to the original procedures.