FORCE QNH?

[SirToppamHat]

Boring one this - sorry!

Looking for input from those of you out there who are lucky enough to spend time wizzing around at low-level.

What do you understand by the term 'Force QNH'? Why do we have such a thing? Who decides what the Force QNH is? What does it achieve? You get the idea - I am not looking for a book definition.

I am hoping that because this is an anonymous thread, people will be honest about this, and if it starts to get too banterish I will delete this post to kill the thread.

Regards

STH

[L J R]

The UK operates on dozens of altimeter settings (QFE/QNH, RPS, Std Setting, 1013, 29.92.........) ask anyone who actually knows the right one to be on a the right time - and you will get dozens on answers (not the point of this question). If a force QNH is nominated, at least ALL in the package will operate on the same QNH (no matter how it was derived) to ensure that the deconfliction plan (in altitude at least) willl work.


In an ideal world, it will be promulgated safely, whilst ensuring that it is the lowest actual, or forecast QNH in an operating area or areas to ensure that MSA as well as deconfliction heights are valid.

[Not Long Here]

Force QNH has been in use for years in the maritime world and enabled all airborne assets within the AOR to use the same setting. Normally promulgated by the ACU of that force and included in the Optask Air.

[SirToppamHat]

The issue of separation from other elements of an exercise (ie the ability to fly in blocks/sanctuaries etc) is self evident.

As you state LJR, if forecasting for the next 3 hours in relatively stable met conditions, you would want a FQNH which is expected to be as low as the lowest anticipated RPS across all areas in which an exercise is taking place. This will provide the separation against other participating elements and should increase safety by additionally providing for separation from land or sea.

In a couple of exercises recently, I have seen FQNHs promulgated that are significantly (as much as 8 millibars) higher that the Met Office RPS forecast for the current and next hour. Assuming that you also have access to the RPSs for the areas you are going into, would you accept this as reasonable?

For those not familiar, one mb = 30 feet (ish), so 8mb equates to about 240 ft. An ac showing 300ft on a pressure altimeter with 1010set will be about 240ft closer to the sea than one with 1002 set on the altimeter sub-scale. I accept that those with radalts may well use these instead, and I am not suggesting anyone would descend to LL in IMC solely on the basis of the FQNH, but the above scenario seems to me to be not unly unsafe, but also easily avoidable. I understand that when this issue has been brought to the attention of those deciding the FQNH, on more than one occasion, individuals have been told to wind their necks in.

How do you feel about this?

STH

[BEagle]

Very, very important and something we insisted upon during GW1. With dozens of strike aicraft and tankers from all across the coalition bases joining up in the dark, deconfliction from eachother, or correctly assigned tanker/receiver levels mandated a common QNH.....

It worked very well indeed; no mid-airs or close calls with everyone on the FQNH!

[Ghostflyer]

STH,

My view, not worth much but surely FQNH is only designed for medium level deconfliction. It can never replace the correct RPS/Rad Alt/TFR for descent into low level. As you suggest, using the lowest possible would be ultra safe but also if you were using too low an RPS you often wouldn't get below the clag. As long as all players are aware of the reasoning, it shouldn't be to the detriment of safety. I always thought the transitioning between different altitude blocks with different evasion criteria was a far more challenging event.

[PLE Always]

Allo,

There was a reference to using the RADALT at lower level (Read MPA/Helo ASW altitudes) however this has been removed as far as I can tell.

This leaves us at low altitude carrying out vertical separation based on a Force QNH and the BAROALT whilst actually flying off the RADALT for operational and safety reasons. It raises all sorts of questions but is just a reality of the job.

My thoughts are, be suspect of arranged separation until you suss out who you're working with and avoid direct overflight unless operationally necessary. In my experience working with the big machine (Coalition) clarification/resolution on this issue will unfortunately never come.

I have seen Force QNH's out to lunch and changed by the ACU at our suggestion/prompting.

Trust no bas...d and look out for yourself. :uhoh: (Healthy respect vice Paranoia)

PLE..

[Pontius Navigator]

Very rusty on this (old metal in sea environment <g>) but when operating in the DAS role with ponys it was essential we were all on the same hymn sheet. When operating in deep field (pre-AEW) then we would be on our tod and able to conduct MAD at MOA based on Rad Alt. Clearly, with FQNH, we could give other units our pressure altitude which had nothing to do with terrain avoidance.

Similarly, and now I am speculating, a FJ ac operating in a formation at ULL would be flying either visual or TFR. If asked for an altitude then they could give the QNH altitude but, this would have nothing to do with terrain avoidance but AA deconfliction.

As someone else said, FQNH is surely for medium level, rather it is for passing your actual pressure level.

[Hoots]

During a recent exercise Force QNH was used by the Eagle Safety team who were manned for a duration before and after a raid. Therefore all checking in and monitoring this frequency should be using the same QNH. The Force QNH was updated hourly. Outside this timespec, when only MPA and Helo's operating the ships Air Controllers provided the QNH setting. However, if ships are some distance away from each other then they may be giving different QNH's to different aircraft. But the Air to Air freq in use should allow others to deconflict, providing they have been briefed on each others movements. Maybe easier just everyone using the RPS.

[vecvechookattack]

I've had a long a vast career buggering about with FQNH....you are all correct in vary degrees....FQNH is great when you are part of a package and you are all singing of that hymn sheet, but there are many profiles (Particularly when flying the Lynx) that we need to be at RADALT height...ASW for instance. So there are the Bags / Pingers flying around on FQNH and the Lynx boys are on their Radalts...Always a tricky one that one

[HEDP]

Am I being a bit slow here or what (opens floodgates),

A Lynx can quite happily sit with FQNH on the bar alt and 'operate' with relation to rad Alt. They are two different altimeters therefore can be used simultaneously!

Or am I missing something here?

HEDP

[Big Toe]

I've had a long a vast career buggering about with FQNH....you are all correct in vary degrees....FQNH is great when you are part of a package and you are all singing of that hymn sheet, but there are many profiles (Particularly when flying the Lynx) that we need to be at RADALT height...ASW for instance. So there are the Bags / Pingers flying around on FQNH and the Lynx boys are on their Radalts...Always a tricky one that one
Obviously not vast enough to haul in that that pinger flying around the Datum where you're doing your wingovers is also operating on RADALT :ugh: . Obvious if you consider thew height accuracy required when going into the dip form 200-odd feet.:rolleyes:

[vecvechookattack]

Big toe is of course absolutely correct...You have to remember that when I started my ASW training we didnt have such clever things as Radalts...

[JanBoy]

Speak to the Navy ATC boys (and girls) at Plymouth (Mil) - HMS Drake. They use the 'Western QNH' as a combined QNH for their DA's and it works fine. The datum is from the barometric pressure indicator on the wall in Air Ops as far as I remember.

[SirToppamHat]

OK, I see where you're all coming from - Force QNH may have originated with the RN (though I may be wrong), and it is clear that the helo drivers amongst the posters here are very familiar with it and its limitations.

In recent years, though it has spread from being used on JMCs (which I think is where I first came across the term) to other large exercises and now some relatively small ones.

I can see why people have the idea that it is useful for medium level (5k+) work in the UK, and that it's less useful at LL, but if you are only working above 5k, and not undertaking ACT, why not set the SAS? The great thing about FQNH as a concept is that it SHOULD work at all levels below FL245, to allow for deconfliction of all players in an area. However, and this is the crux of the matter, the chosen FQNH MUST be as low as the RPSs the Met Office can predict for the period of the activity.

What I and some of my colleagues are concerned about is that the Force QNH seems sometimes to be decided by people who appear to pay scant regard to the forecast RPSs, and even brief that the Force QNH 'should not be used for separation from the surface', whereas, in practice, that should be one of its main functions. My advice to anyone receiving a Force QNH about which they have any doubts is to compare it with the RPS - if it is higher than any of the forecast RPSs for the areas in which you will be working question it - PLEASE! I know controllers who have done so in the past and been told to shut up. Then again we have never lost a controller yet, so why should we be worried? We care!

STH