Airway question

[funfly]

If you have an airway marked on the (half mil) map which has a lower altitude marked e.g. +3000, Should this be negociated using Regional Pressure?
I appreciate that if an airfield lies within this airspace (sorry, underneath it!) then it may be sensible to use the airfield QNH but I need to know the factual position, i.e. are lower airway limits, when written in feet, based on the assumption that people wishing to pass under these airways are OK using a regional setting?

[IO540]

No, you are supposed to obtain the QNH from the ATC unit that serves that airspace, or one nearby.

RPS can be miles out, which is why only the UK uses it :O Just like QFE, another crazy old oddity.

[Stan Evil]

IO540 is absolutely correct.

Markflyer6580 is not.

[High Wing Drifter]

are lower airway limits, when written in feet, based on the assumption that people wishing to pass under these airways are OK using a regional setting?
No, when expressed a altitude rather than FL, you are recommended to use the a nearby aerodrome QNH, not the regional.

http://www.ais.org.uk/aes/pubs/aip/pdf/enr/20107.PDF

[markflyer6580]

I agree! I have noticed the military like QFE on a matz crossing though.

[funfly]

UKAIP ENR 1-7-2
5.2.2
In flight or below 3000' amsl pilots may use any desired setting. However pilots flying beneath a TMA or CTA should use the QNH of an aerodrome situated beneath that area when flying beneath the transition zone.....Pilots in flight at or below 3000' on an advisory route should set altimeters to the appropriate Regional Pressure Setting.

This seems to indicate that within a TMA or CTA the aerodrome QNH must be used but outside of this regional pressure should be used.

It may be safer or even more sensible to fly at a nearby airfield QNH even if you are not within their TMA or CTA but I am looking for clarification of the true position - If I was flying at 2,900' on the Regional Pressure Setting and 3,200' on the local aerodrome QNH would I oficially be busting the airway if it was marked on the chart +3000' ?

[Chilli Monster]

Funfly

Advisory routes (as quoted by you) are not Class 'A' airways - they're Class 'F'. As such you can fly in them VFR without a clearance anyway, IFR if you don't want to participate in the ATC advisory service. Therefore it doesn't really matter what your altimeter is set on.

If an Airway (Class 'A') is notified with a base level of an altitude then the chances are it's associated with a TMA or it's adjacent to an airfield to allow Commercial aircraft to step descent into it. That unit will be providing separation based on it's own QNH. That's what you should be using.

(Bear in mind 3000ft on the regional is always higher than 3000ft on an airfields QNH - I know which one I'd use to make sure I stay outside ;) )

[DFC]

The answer lies on the availability of Aerodrome QNH. In the case of a TMA, or a CTA where the base is defined as an altitude, there will always be an associated aerodrome or aerodromes. You should use the aerodrome QNH to ensure that you do not infringe the CTA.

With an Airway, the situation is different. The Airway could be a considerable distance from an aerodrome reporting QNH. Thus aerodrome QNH may not always be available. Consequently, one can use the Regional PressureSetting (RPS) but it is recomended that an aerodrome QNH is used if available.

Airways will have a base defined as an altitude in two circumstances;

1. Simply a base altitude e.g. 3000 - FL245. Easy one there.

or

2. Complicated - a base defined as a Flight level but an assurance that the base will never be lower than a certain altitude. e.g. FL65 - FL245 minimum ALT 5500.

ATC will use the RPS to determine the minimum level available on the airway (500ft above the base) and Sectors where this happens will have a chart of RPS value against minimum level available.

Simple answer - get rid of RPS unless you can find an airway in the UK defined as a base altitude and more than 50nm from where one can obtain an aerodrome QNH. It can't be that safe when even the AIP can't be relied on for an exact answer.

Regards,

DFC

[funfly]

Thank you guys - my brain has just clicked into gear.
All started because I needed to fly under an airway which specified +3000'. I questioned whether to use a local airfield QNH or the Barnsley and looked to the half mil map to see if it told me:confused:
Of course 3000' is a fixed point in the sky 3000' above the level of the sea so it doesn't matter what you use to measure this, even a piece of string! Pressure doesn't come into it at all.
So if you want to find this point you use the best instrument you can (the altimeter) and you use the pressure setting most applicable to the bit of sky you are in.
So local airfield QNH is as good as you can get here.
Don't you hate it when you demonstrate your dumbness in public:uhoh:

[EGBKFLYER]

Yeah - embarrassing but it gives the rest of us chance to learn something without looking dumb :ok: :}