Airways with base FL35 and low QNH

[BackPacker]

Following a discussion in another thread (http://www.pprune.org/forums/showthread.php?p=3481068#post3481068):

There are a few airways over the Channel with base FL35, while the transition altitude is 3000 feet. In a situation with low air pressure, FL35 may well be below 3000 feet.

According to my Air Law book, flight levels always start above the transition altitude. With low pressure, FL35 therefore may not exist. At the very least, it is not available for flying.

But what happens to the airway if the base is defined as FL35? Will the base really, formally, legally, drop below the transition altitude of 3000 feet? It would be more logical (and safer) if the base would never be lower than the transition altitude. Plus, the lowest flight level in that airway is not available anyway, so who would care?

My air law book doesn't give an answer. Anybody else knows the details?

[PAPI-74]

Trans Alt: At or below is on QNH

Trans level: At or above is on Standard (1013.25)

In answer to your question, with a low QNH the trans alt would stay the same. It is printed on the IFR plates as unchangable.

The trans level is given by ATC usually, just for that reason. You can use the bottom 500' for VFR, so if an airway started at FL035, the first available level is FL040, giving a buffer to the IFR traffic, but as it is controlled airspace, you will need a service.

Having a flexable trans level gets over the problem of low pressures, while keeping a fixed trans alt maintains safe terrain sep. You may find that it is several thousand feet above the trans alt, or 500'.

JD

[gcolyer]

3000ft is not the defacto transition altitude, although it is the common transition altitude.

Barometric pressure will determine what FL's are available. For instance:

QNH=996mb

FL30 would not be available as the altitude would actualy be 2490ft (based on 30ft per millibar).

This means that you would need to fly the next available FL for direction of travel. In this instance if where planning to fly FL30 we would need to fly FL40.

As for transition altitude not even FL35 would be available as the altitude for FL35 would be 2990ft. This means the tranisition altitude will be 4000ft and no FL's below FL40 are available.

[bookworm]

Good question. Here's the note on R41:

"AWY base is FL 35 or 3014 ft amsl, whichever is higher. Consequently, when low atmospheric pressure prevails, FL 40 may not be available as a cruising FL."

BTW there's some discrepancy in international usage of the phrase "transition level". In some places it means the lowest level with 1000 ft of separation from the transition altitude. In the UK, it's simply the lowest multiple of 5 flight level above the TA. (E.g. QNH 1014, transition level is FL30).

[bookworm]

QNH=996mb
...
As for transition altitude not even FL35 would be available as the altitude for FL35 would be 2990ft.

On a point of utter pedantry, FL35 is in fact available with a QNH of 996, as the conversion is not exactly 30 ft/mbar, but a little less.

[Brooklands]

This means that you would need to fly the next available FL for direction of travel. In this instance if where planning to fly FL30 we would need to fly FL40.
Errrm, I think you mean FL50. Flight levels at odd thousands of feet are for tracks between north and east*. Flight levels at even thousands of feet and for tracks between South and west.
Brooklands
*Using UK quadrantal rules

[gcolyer]

On a point of utter pedantry, FL35 is in fact available with a QNH of 996, as the conversion is not exactly 30 ft/mbar, but a little less.

Ok ok it is actualy 27ft per millibar so you are right FL35 would be available. However all the book I have read and the CAA exams go by 30ft per millbar

God pedantry:rolleyes:


Errrm, I think you mean FL50. Flight levels at odd thousands of feet are for tracks between north and east*. Flight levels at even thousands of feet and for tracks between South and west.
Brooklands
*Using UK quadrantal rules


DOH! school boy error.:bored:

But the transistion altitude would still be 4000ft

[bookworm]

*Using UK quadrantal rules

The quadrantal rule doesn't actually apply in controlled airspace, so in the airway you simply fly at the assigned level. That's usually odds for eastbounds and evens for westbounds, but it's perfectly possible to request the FL40 for an eastbound flight.

Ok ok it is actualy 27ft per millibar

Well I cheated -- there's a table at the back of MATS Part 1. ;)

[IO540]

If some old-timers here find this stuff confusing, makes you wonder what it's like for the average PPL.

I would have never thought of the airway base converting itself from a FL to an altitude, unless there is a clear note on the chart right next to the FL label.

The transition level tends to be ignored by most pilots, and ATC often give you seemingly arbitrary levels. One day you can get 4000ft, on another day you can get FL40. I ignore the quad levels anyway because most of the time they are overrriden by MSA/airspace considerations.

The pressure change rate isn't 30 or 27ft per mb; it is a continuously varying ratio. The 30ft/mb we have all been taught in the PPL is meaningful only at very low levels of zero to say 3000ft.

[bookworm]

I would have never thought of the airway base converting itself from a FL to an altitude, unless there is a clear note on the chart right next to the FL label.

There always used to be exactly that where there was a minimum altitude. I don't have a half-mil to hand.

The pressure change rate isn't 30 or 27ft per mb; it is a continuously varying ratio. The 30ft/mb we have all been taught in the PPL is meaningful only at very low levels of zero to say 3000ft.

Yes but bear in mind that the subscale setting is always at the sea-level end of the column of air. So changing the subscale from 1013 to 1014 will add about 30 feet whether you're at 1000 ft or FL390. Changing the subscale from any-reasonable-QNH to any-reasonable-QNH + 1 will always add about 30 feet.

[BackPacker]

Here's the note on R41:

"AWY base is FL 35 or 3014 ft amsl, whichever is higher. Consequently, when low atmospheric pressure prevails, FL 40 may not be available as a cruising FL."

Bookworm, where did you find that note? The UK ICAO half-mil I have here has nothing on R41 in the notes, and on the map itself only says FL35+.

[bookworm]

UK AIP ENR 3.1, which formally defines the airway.

[englishal]

You can use the bottom 500' for VFR, so if an airway started at FL035, the first available level is FL040, giving a buffer to the IFR traffic, but as it is controlled airspace, you will need a service
Which service would that be then for a VFR pilot in the class A airway?

I don't think the confusion is about what flight level is available, but the fact that FL35 can actually be LOWER than A3000'. So if you're flying along nicely at 2995' on QNH you could be busting a class A airway - even though you won't meet anyone in it because that FL is not available.

All this variable transition altitude stuff is a mess. Look just north of R whatever (old N866) and you will see Solent airspace from A3500 to FL55. IN theory you could be flying along A3000 on solent QNH below their airspace, then as soon as you "leave" their airspace and head underneath ol N866 you are busting the Class A for the reasons above.

Nuts.:ugh:

[oceans 11]

As a controller at swanwick who works this airspace, we are told when the pressure is too low ( I think 995mb) the min FL we can use is FL50. It doesn't affect us too much just the Trilander that flies to Alderney and back.

It can be confusing and I'm glad we have a table that lets us know whats right.

[Jumbo Driver]

But what happens to the airway if the base is defined as FL35? Will the base really, formally, legally, drop below the transition altitude of 3000 feet? It would be more logical (and safer) if the base would never be lower than the transition altitude. Plus, the lowest flight level in that airway is not available anyway, so who would care?


BackPacker,

I'll have a stab at a definitive answer ...


1. The base of airway R41 (THRED to ORTAC) can never be below the Transition Altitude of 3000' because of the note in the Remarks column at UKAIP ENR 3.1 (page ENR 3-1-1-86R), which reads:

AWY base is FL 35 or 3014 ft amsl, whichever is higher. Consequently, when low atmospheric pressure prevails, FL 40 may not be available as a cruising FL.


2. In general terms, the base of an airway cannot normally be below 3001 ft because of UKAIP ENR 1.1.1 para 1.2 (page ENR 1-1-1-1), which reads:

1.2 Except where stated otherwise the width of an Airway is 5 nm either side of a straight line joining each two consecutive points shown in column 1 of the table. Upper ATS Routes and Advisory Routes have no declared width but for the purposes of ATS provision are deemed to be 5 nm either side of a straight line joining each two consecutive points. The vertical extent is shown in column 3 of the table. Where lower limits of Airways and Advisory Routes are defined as Flight Levels an absolute minimum altitude of 3001 ft applies unless otherwise stated in column 3 and the minimum cruising level shown in column 5 may not always be available.


3. The lowest usable FL is defined by UKAIP ENR 1.1.1 para 6.1.1 (page ENR 1-1-1-7), which reads:

6.1.1 Where the lower limit of a section of an Airway is defined as a Flight Level and therefore varies in height, an absolute minimum altitude applies. This minimum altitude for the Airway base is at least 1000 ft above any fixed obstacle within 15 nm of the centre-line. The lowest usable level will always be at least 500 ft above the Airway base, thus providing not less than 1500 ft terrain clearance within 15 nm of any position on the centre-line of the Airway.


4. There is a Flight Level Graph at UKAIP ENR 1.7 (page ENR 1-7-5), which can be used to read off a FL against Altitude and QNH. However, a much easier method is to use the Pressure Setting Tables at CAP 493 (MATS Pt 1), Appendix A (page 1) which, with a TA of 3000', determines TL as follows:

QNH = 1050-1032, TL = FL25
QNH = 1031-1014, TL = FL30
QNH = 1013-996, TL = FL35
QNH = 995-978, TL = FL40
QNH = 977-960, TL = FL45
etc.


5. Conclusion:

a) The lower limit of airway R41 is defined as FL35 or 3014', whichever is higher.
b) The lower limit of airway R41 is FL35 when QNH is 996 or higher, and 3014' otherwise.
c) The lowest usable FL within the airway is:

FL40 when QNH is 996 or higher
FL45 when QNH is 995-978
FL50 when QNH is 977-960
etc.


Hope all this helps ...



JD
:)

[BackPacker]

Thanks, everyone. The most important conclusion with regards to R41 is that if you fly below the transition altitude (3000'), so on QNH, the airway will never 'descend on you' if the pressure is low.