Altitude or FL under VFR?
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Altitude or FL under VFR?
I was just planning my route to Duxford for April, and got to thinking about what altitudes I may be using. I must admit that I've never been completley sure when I should use Flight Levels and when it's OK to fly above 3000ft still on regional QNH. I fully understand the difference between the two (QNH/1013.2), when to apply them (transision level/altitude), just not if and when I have to use them.
Yes, I know I should know this, but I don't, and I know I'm not alone. This always seems to cause confusion when I talk to people about it so please enlighten us. I've re-read the relevant Thom section, which goes into a lot of detail but doesn't really answer the question. Anyway, here's how I understand it - am I correct?
When flying VFR in uncontrolled airspace I can use any altimiter setting I want and fly at any altitude regardless of any quadrangle rules etc. (subject obviously to 1500ft/500ft rules) and VFR minima).
When using VFR above the transition altitude I can either remain on QNH and fly at 'x thousnad feet' or choose to set 1013.2 on the altimiter and fly flight levels. I can fly at any level but compliance with the quadrangle rule is recommended.
If for some reason I were flying IFR in VMC, flight levels would be compulsary above the transision altitude as would compliance with the quadrangle rule.
Hope this makes sense. Basically what I'm asking is, what do most PPL's decide to do when flying at around 4000 to 6000 feet - QNH or flight levels, an why?
Thanks for the help
Here's hoping the weather is good enough to get up to those dizzy heights. 
Yes, I know I should know this, but I don't, and I know I'm not alone. This always seems to cause confusion when I talk to people about it so please enlighten us. I've re-read the relevant Thom section, which goes into a lot of detail but doesn't really answer the question. Anyway, here's how I understand it - am I correct?
When flying VFR in uncontrolled airspace I can use any altimiter setting I want and fly at any altitude regardless of any quadrangle rules etc. (subject obviously to 1500ft/500ft rules) and VFR minima).
When using VFR above the transition altitude I can either remain on QNH and fly at 'x thousnad feet' or choose to set 1013.2 on the altimiter and fly flight levels. I can fly at any level but compliance with the quadrangle rule is recommended.
If for some reason I were flying IFR in VMC, flight levels would be compulsary above the transision altitude as would compliance with the quadrangle rule.
Hope this makes sense. Basically what I'm asking is, what do most PPL's decide to do when flying at around 4000 to 6000 feet - QNH or flight levels, an why?
Thanks for the help
Here's hoping the weather is good enough to get up to those dizzy heights.
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Hey knobby gb,
I was planning a flight from Blackpool to Carlisle (over the Lake District) and was faced with the same dilemma (I'm a vanilla PPL by the way!) In the UAS, it was standard procedure to set 1013 when above transition, however due to the nature of the sorties, we never applied the quadrantal rule.
I have decided that whenever on a specific nav route (i.e. not sightseeing around the local area) that I would fly in accordance with flight level and the quadrantal rule when above transition. It means you have to do a bit more planning (i.e. available flight levels on the day/pressure alt etc), but it's neater. However, it's pretty rare in my experience that a light a/c flies above 3000ft anyway. There's no real reason why you shouldn't go high, infact when it's possible, you should, but in my opinion it's just good practice to fly FLs.
I was planning a flight from Blackpool to Carlisle (over the Lake District) and was faced with the same dilemma (I'm a vanilla PPL by the way!) In the UAS, it was standard procedure to set 1013 when above transition, however due to the nature of the sorties, we never applied the quadrantal rule.
I have decided that whenever on a specific nav route (i.e. not sightseeing around the local area) that I would fly in accordance with flight level and the quadrantal rule when above transition. It means you have to do a bit more planning (i.e. available flight levels on the day/pressure alt etc), but it's neater. However, it's pretty rare in my experience that a light a/c flies above 3000ft anyway. There's no real reason why you shouldn't go high, infact when it's possible, you should, but in my opinion it's just good practice to fly FLs.
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You mean USA? It was flying over there recently (where quad rule is mandatory over 3000ft) that made me think of this really. Much simpler because a light a/c will (nearly) always be on QNH - QFE doesn't 'exist' as a concept and transition altitude is much higher so not an issue.
I agree that I like to get as much height as possible in decent weather. A) because if anything goes wrong it gives you more time to plan and B) just for the novelty!
I agree that I like to get as much height as possible in decent weather. A) because if anything goes wrong it gives you more time to plan and B) just for the novelty!
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Nav routes I tend to use FL's if I want to fly above TA
Gen handling - generally altitudes but change to FLs if operating on top of a broken layer.
I have to say I find it very difficult to understand the CAA recommendation for VFR traffic to fly quadrantals:
a) There are so many aircraft out there operating on regional QNH's, airfield QNH's, 1013 and QFE's that I can't imagine a significant reduction in collision risk exists by VFR pilots flying quadrantals
b) There's so much scope for a low hour PPL to make a mistake by changing to 1013, particularly in attempting to fly below controlled airspace. I know the regional QNH carries the same risk, but at least its usually closer to the aerodrome QNH.
PS best radio I've heard for a long time:
G-CD: "XXX RADAR, Microlight G-CD at FL three thousand five hundred"
XXX Radar: "Confirm 1013 set and report FL"
G-CD: "Affirm 1013 set, FL 3,500 feet on 1020"
Gen handling - generally altitudes but change to FLs if operating on top of a broken layer.
I have to say I find it very difficult to understand the CAA recommendation for VFR traffic to fly quadrantals:
a) There are so many aircraft out there operating on regional QNH's, airfield QNH's, 1013 and QFE's that I can't imagine a significant reduction in collision risk exists by VFR pilots flying quadrantals
b) There's so much scope for a low hour PPL to make a mistake by changing to 1013, particularly in attempting to fly below controlled airspace. I know the regional QNH carries the same risk, but at least its usually closer to the aerodrome QNH.
PS best radio I've heard for a long time:
G-CD: "XXX RADAR, Microlight G-CD at FL three thousand five hundred"
XXX Radar: "Confirm 1013 set and report FL"
G-CD: "Affirm 1013 set, FL 3,500 feet on 1020"
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knobbygb
Rather than refer to the Thom section try the appropriate section of the ANO or AIP. I don't have one immeduately to hand so can't give you the definitive however I think you'll find that whilst it may not say 'compulsory' you will probably find the word 'should'.
This always brings you back to the case of although perhaps not citing something as being mandatory, if you don't heed the advice and something happens as a result you will be expected to have a good reason why.
Rather than refer to the Thom section try the appropriate section of the ANO or AIP. I don't have one immeduately to hand so can't give you the definitive however I think you'll find that whilst it may not say 'compulsory' you will probably find the word 'should'.
This always brings you back to the case of although perhaps not citing something as being mandatory, if you don't heed the advice and something happens as a result you will be expected to have a good reason why.
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Neither the altimeter setting nor the quadrantal rule (UK specific the rest of the world uses the semicircular rule) are things you can use at will.
They *are* compulsory.
So above the TA you have to use 1013 and FL below that the QNH.
Yet another thing that does not get taught properly over here as everyone just bashes the circuit rather than flies some sorties and gets out and about. The very thing that most people want to get trained for really.
The fact that so many people give up after their PPL and the 50 odd hours of bashing the local area is that they are so ill equipped for flying anywhere.
(Yes indeed a hobby horse of mine!)
FD
TA varies depending where you are in the country.
They *are* compulsory.
So above the TA you have to use 1013 and FL below that the QNH.
Yet another thing that does not get taught properly over here as everyone just bashes the circuit rather than flies some sorties and gets out and about. The very thing that most people want to get trained for really.
The fact that so many people give up after their PPL and the 50 odd hours of bashing the local area is that they are so ill equipped for flying anywhere.
(Yes indeed a hobby horse of mine!)
FD
TA varies depending where you are in the country.
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Sorry, UAS meant University Air Squadron!! In the UAS, it was standard practise to set QNE above transition. Back to the point about getting as high as you can (i.e. FL): More gliding range and time if the engine should quit, you're out of the way of fast jets doing low level stuff (especially over the Lake District and the like), there's less light traffic up there, so lower risk of collision (to a certain extent), the winds are stronger (which could be used to your advantage) and you get a better TAS...the list goes on!! Grief, now I'm wondering why I ever stuck at 2000ft!!
Why do it if it's not fun?
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FD,
Do you have a reference for that? Because my understanding was that setting 1013.2 and flying according to the quadrantal rule are recommended above the transition level, but not compulsary, for a VFR flight.
FFF
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Do you have a reference for that? Because my understanding was that setting 1013.2 and flying according to the quadrantal rule are recommended above the transition level, but not compulsary, for a VFR flight.
FFF
-------------
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You should always use FLs when enroute above the transition altitude, even if VFR, unless the pressure is sufficiently low that the flight level is below the TA and therefore unavailable (e.g. FL35 with regional 990 mb puts you at 2800' QNH). Remember that although in the UK open FIR the TA is 3000', the various portions of controlled airspace have their own TAs which can be considerably higher. One important exception is when you have controlled airspace above you whose base is defined as an altitude (e.g. London or Manchester TMAs). Then you should set QNH of the controlling authority and fly altitudes.
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Flying Dutch
Sorry - but wrong. Flight Levels and quadrantal rule are only mandatory for IFR flight above the transition altitude. If you're VFR you can fly at whatever level you like because at the end of the day you are just that - VFR and so have the right (and sometimes the need) to change your level to stay VFR (Rule 30 is the relevant rule for IFR, there is nothing similar in the VFR rules - 24 to 27)
BUT
If you're flying close to the base of CAS you should be on the same pressure setting that the base is measured from - 1013 if it is FL's, the AERODROME QNH (not the regional, this will put you too high) that the base is again measured from (London QNH in the case of the the London TMA, respective aerodrome QNH if passing under an airfield CTA.
Sorry - but wrong. Flight Levels and quadrantal rule are only mandatory for IFR flight above the transition altitude. If you're VFR you can fly at whatever level you like because at the end of the day you are just that - VFR and so have the right (and sometimes the need) to change your level to stay VFR (Rule 30 is the relevant rule for IFR, there is nothing similar in the VFR rules - 24 to 27)
BUT
If you're flying close to the base of CAS you should be on the same pressure setting that the base is measured from - 1013 if it is FL's, the AERODROME QNH (not the regional, this will put you too high) that the base is again measured from (London QNH in the case of the the London TMA, respective aerodrome QNH if passing under an airfield CTA.
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Sorry about the length of this post, but attached are the words from the AIP (version downloaded today, so it has a validity of no longer than 24 hrs!) - usual health precautions apply
:
For those wot are registered with the AIS site, the link is below:
AIP En-Route Altimeter Settings Link
My reading of this is that under IFR or in controlled airspace, there is a mandatory use of FLs and quadrantal rules (Europe, below FL245) above Transition Altitude. If one is VFR outside controlled airspace, this is very vaguely covered in the AIP but it appears that there is no mandatory requirement for the use of FLs.
:
5 Detailed Procedures
5.1 Take-off and climb
5.1.1 A QNH altimeter setting is given with the taxiing clearance prior to take-off.
5.1.2 At UK aerodromes the designated location for pre-flight altimeter checks is the apron.
5.1.3 For all major UK aerodromes, the apron elevation (or the elevation of various parts of an apron where there is significant
variation between them) has been determined and the value is displayed in the flight clearance office at the aerodrome concerned. It is also given at AD 2.8.
5.1.4 Within Controlled Airspace a pilot should set one altimeter to the latest Aerodrome QNH prior to take-off. While flying at, or below, the Transition Altitude vertical position will be expressed in terms of altitude based upon the Aerodrome QNH. When cleared for climb to a Flight Level, vertical position will be expressed in terms of Flight Level, unless intermediate altitude reports have been specifically requested by Air Traffic Control.
5.1.5 Outside Controlled Airspace, a pilot may use any desired setting for take-off and climb. However, when under IFR, vertical position must be expressed in terms of Flight Level on climbing through the Transition Altitude.
5.1.6 Pilots taking-off at aerodromes beneath Terminal Control Areas and Control Areas should use aerodrome QNH when flying below the Transition Altitude and beneath these Areas, except that the aerodrome QFE may be used when flying within the circuit. It may be assumed that for aerodromes beneath the same TMA or CTA the differences in their QNH values are insignificant.
5.2 En-route
5.2.1 Within Controlled Airspace
5.2.1.1 At and above the transition level and during en-route flight the aircraft should be flown at Flight Levels. The latest and most appropriate Regional Pressure Setting value is to be used for checking terrain clearance in flight. Aircraft flying in a Control Zone or TMA at an Altitude at or below the Transition Altitude will be given the appropriate QNH setting in their clearance to enter the Zone/TMA.
5.2.2 Outside Controlled Airspace
5.2.2.1 In flight at or below 3000 ft 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 below the Transition Altitude. It may be assumed that for aerodromes beneath the same TMA or CTA, the differences in the QNH values are insignificant. References to vertical position in flight plans and communications with ATC are to be expressed in terms of altitude. Pilots in flight at or below 3000 ft amsl on an Advisory Route should set altimeters to the appropriate Regional Pressure Setting.
5.2.2.2 When flying under IFR above the Transition Altitude pilots must have 1013.2 mb set on an altimeter and conform to the Quadrantal Rule in accordance with ENR 1.7.3, paragraph 6.1 when flying at/below FL 245, and the Semi-circular Rule when above FL 250. When flying under VFR pilots are recommended to conform to the Quadrantal Rule and Semi-circular Rule as appropriate. The latest and most appropriate lowest forecast Regional Pressure Setting value should be used for checking terrain clearance.
5.3 Approach and Landing
5.3.1 When an aircraft is descended from a Flight Level to an Altitude preparatory to commencing approach for landing, ATC will pass the appropriate aerodrome QNH. On vacating the Flight Level, the pilot will change to the aerodrome QNH unless further Flight Level vacating reports have been requested by ATC, in which case, the aerodrome QNH will be set following the final Flight Level vacating report. Thereafter, the pilot will continue to fly on the aerodrome QNH until established on final approach when QFE or any other desired setting may be used. However, ATC (except at certain military aerodromes (See Note)) will assume that an aircraft is using QFE on final approach when carrying out a radar approach and any heights passed by the radar controller will be related to QFE datum. A reminder of the assumed setting will be included in the RTF phraseology. To ensure the greatest possible degree of safety and uniformity, it is recommended that all pilots use QFE but, if the pilot advises that he is using QNH, heights will be amended as necessary and 'Altitude' will be substituted for 'height' in the RTF phraseology. It should be noted that the Obstacle Clearance Height is always given with reference to the aerodrome or threshold elevation.
5.3.2 If it is known that a particular company uses, or a pilot has clearly indicated that he will use QNH during the final approach, the controller may omit QFE and substitute QNH and the relevant elevation in the appropriate messages.
5.3.3 Vertical positioning of aircraft during approach will, below transition level, be controlled by reference to Altitudes and then to heights. The Transition Altitude is not normally given in the approach and landing clearance.
5.3.4 Pilots landing at aerodromes beneath Terminal Control Areas and Control Areas should use aerodrome QNH when flying below the Transition Altitude and beneath these Areas, except that the aerodrome QFE may be used when flying within the circuit. It may be assumed that for aerodromes within the boundary of the same TMA or CTA, differences in their QNH values are insignificant.
5.1 Take-off and climb
5.1.1 A QNH altimeter setting is given with the taxiing clearance prior to take-off.
5.1.2 At UK aerodromes the designated location for pre-flight altimeter checks is the apron.
5.1.3 For all major UK aerodromes, the apron elevation (or the elevation of various parts of an apron where there is significant
variation between them) has been determined and the value is displayed in the flight clearance office at the aerodrome concerned. It is also given at AD 2.8.
5.1.4 Within Controlled Airspace a pilot should set one altimeter to the latest Aerodrome QNH prior to take-off. While flying at, or below, the Transition Altitude vertical position will be expressed in terms of altitude based upon the Aerodrome QNH. When cleared for climb to a Flight Level, vertical position will be expressed in terms of Flight Level, unless intermediate altitude reports have been specifically requested by Air Traffic Control.
5.1.5 Outside Controlled Airspace, a pilot may use any desired setting for take-off and climb. However, when under IFR, vertical position must be expressed in terms of Flight Level on climbing through the Transition Altitude.
5.1.6 Pilots taking-off at aerodromes beneath Terminal Control Areas and Control Areas should use aerodrome QNH when flying below the Transition Altitude and beneath these Areas, except that the aerodrome QFE may be used when flying within the circuit. It may be assumed that for aerodromes beneath the same TMA or CTA the differences in their QNH values are insignificant.
5.2 En-route
5.2.1 Within Controlled Airspace
5.2.1.1 At and above the transition level and during en-route flight the aircraft should be flown at Flight Levels. The latest and most appropriate Regional Pressure Setting value is to be used for checking terrain clearance in flight. Aircraft flying in a Control Zone or TMA at an Altitude at or below the Transition Altitude will be given the appropriate QNH setting in their clearance to enter the Zone/TMA.
5.2.2 Outside Controlled Airspace
5.2.2.1 In flight at or below 3000 ft 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 below the Transition Altitude. It may be assumed that for aerodromes beneath the same TMA or CTA, the differences in the QNH values are insignificant. References to vertical position in flight plans and communications with ATC are to be expressed in terms of altitude. Pilots in flight at or below 3000 ft amsl on an Advisory Route should set altimeters to the appropriate Regional Pressure Setting.
5.2.2.2 When flying under IFR above the Transition Altitude pilots must have 1013.2 mb set on an altimeter and conform to the Quadrantal Rule in accordance with ENR 1.7.3, paragraph 6.1 when flying at/below FL 245, and the Semi-circular Rule when above FL 250. When flying under VFR pilots are recommended to conform to the Quadrantal Rule and Semi-circular Rule as appropriate. The latest and most appropriate lowest forecast Regional Pressure Setting value should be used for checking terrain clearance.
5.3 Approach and Landing
5.3.1 When an aircraft is descended from a Flight Level to an Altitude preparatory to commencing approach for landing, ATC will pass the appropriate aerodrome QNH. On vacating the Flight Level, the pilot will change to the aerodrome QNH unless further Flight Level vacating reports have been requested by ATC, in which case, the aerodrome QNH will be set following the final Flight Level vacating report. Thereafter, the pilot will continue to fly on the aerodrome QNH until established on final approach when QFE or any other desired setting may be used. However, ATC (except at certain military aerodromes (See Note)) will assume that an aircraft is using QFE on final approach when carrying out a radar approach and any heights passed by the radar controller will be related to QFE datum. A reminder of the assumed setting will be included in the RTF phraseology. To ensure the greatest possible degree of safety and uniformity, it is recommended that all pilots use QFE but, if the pilot advises that he is using QNH, heights will be amended as necessary and 'Altitude' will be substituted for 'height' in the RTF phraseology. It should be noted that the Obstacle Clearance Height is always given with reference to the aerodrome or threshold elevation.
5.3.2 If it is known that a particular company uses, or a pilot has clearly indicated that he will use QNH during the final approach, the controller may omit QFE and substitute QNH and the relevant elevation in the appropriate messages.
5.3.3 Vertical positioning of aircraft during approach will, below transition level, be controlled by reference to Altitudes and then to heights. The Transition Altitude is not normally given in the approach and landing clearance.
5.3.4 Pilots landing at aerodromes beneath Terminal Control Areas and Control Areas should use aerodrome QNH when flying below the Transition Altitude and beneath these Areas, except that the aerodrome QFE may be used when flying within the circuit. It may be assumed that for aerodromes within the boundary of the same TMA or CTA, differences in their QNH values are insignificant.
AIP En-Route Altimeter Settings Link
My reading of this is that under IFR or in controlled airspace, there is a mandatory use of FLs and quadrantal rules (Europe, below FL245) above Transition Altitude. If one is VFR outside controlled airspace, this is very vaguely covered in the AIP but it appears that there is no mandatory requirement for the use of FLs.
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Circuit Basher
I thought the whole point of people asking on a forum was because they wanted it in plain language - great swathes of the AIP being the problem regarding understanding in the first place
I thought the whole point of people asking on a forum was because they wanted it in plain language - great swathes of the AIP being the problem regarding understanding in the first place
Last edited by Chilli Monster; 12th Mar 2003 at 16:46.
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CM
As in many threads various posters have made a stab with varying data, this is interesting, but confusing, so a source document quote is one way of making sure nobody stays confused. This CB has done.
I appreciate you gave the correct answer, but many would have to go back to source documentation anyway to determine which posts were correct.
Perhaps the next version of pprune will have an automarking system, red for wrong info and green for correct info?
As in many threads various posters have made a stab with varying data, this is interesting, but confusing, so a source document quote is one way of making sure nobody stays confused. This CB has done.
I appreciate you gave the correct answer, but many would have to go back to source documentation anyway to determine which posts were correct.
Perhaps the next version of pprune will have an automarking system, red for wrong info and green for correct info?
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CM - reason for long extract (actually, less than 1 page of a 4 page section of the AIP!) was due to fact that a lot of what had already been posted was *partly* right under some circumstances. Danger of posting just a little bit of it or summarizing, there is a danger of personal interpretation coming into it or 'snipping' a bit which is pertinent to the discussion. I'd far rather show the whole context and let the reader make his / her own interpretation than only show a small extract.
IMHO, a judgement call / personal preference for which there is no right / wrong - I can see your point and understand where you're coming from. I just happen to be coming from a different direction!!
IMHO, a judgement call / personal preference for which there is no right / wrong - I can see your point and understand where you're coming from. I just happen to be coming from a different direction!!
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CM you are absolutely right! Pologies for any confusion caused.
I think though that CB was very helpful for posting this integral bit of information as it is a very useful piece.
Don't be so harsh.
FD
I think though that CB was very helpful for posting this integral bit of information as it is a very useful piece.
Don't be so harsh.
FD
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Knobby
A practical viewpoint ....
If you are planning to fly above the transition level and use RIS, I would set 1013 and declare flight levels as it will make it easier for the controller to relate you to IFR traffic that s/he may be working. The quadrantal rule is no prob, just remember to add the mag variation to your true track to determine which quadrant you're in.
Otherwise, I'd use regional QNH. I tend to cruise at slightly 'odd' levels like 3300', in the vain hope that this will keep me away from IFR traffic, but this isn't guaranteed due to altimeter error and relative differences the QNH and the standard 1013.2.
Hope this helps.
A practical viewpoint ....
If you are planning to fly above the transition level and use RIS, I would set 1013 and declare flight levels as it will make it easier for the controller to relate you to IFR traffic that s/he may be working. The quadrantal rule is no prob, just remember to add the mag variation to your true track to determine which quadrant you're in.
Otherwise, I'd use regional QNH. I tend to cruise at slightly 'odd' levels like 3300', in the vain hope that this will keep me away from IFR traffic, but this isn't guaranteed due to altimeter error and relative differences the QNH and the standard 1013.2.
Hope this helps.
To digress only a little
Transferring to Canada and doing my PStar I was concerned that we have the semi rule not the quadrantal. Why is that? Personally I think it looks like a recipe for an accident, am I missing something here?
Transferring to Canada and doing my PStar I was concerned that we have the semi rule not the quadrantal. Why is that? Personally I think it looks like a recipe for an accident, am I missing something here?
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What about terrain avoidance ???
Using 1013 mB above transition altitude and applying the quadrangle rule for VFR flights across the flatlands of southern and eastern England is all very well when the primary concern is avoidance of conflicting traffic. However, terrain across large parts of Wales, Scotland, and northern England pokes up a little bit here and there through 3,000ft.
I will be flying VFR tommorrow in Mid-Wales. My MSA for one leg is 4,300ft (over high ground of 2,907ft). If the cloudbase is similar to today it will be about 3/8 at 5,000. To stay legal / safe it would be impossible to fly quadrangles at 1013 mB. Even if it were possible to comply with the quadrangle rule and use flight levels, I would be much more comfortable flying on regional QNH so that I can equate my altimeter reading to the heights above mean sea level on my chart rather than on a theoretical pressure setting that provides no information to me of a practical nature for terrain avoidance.
Mr W.
I will be flying VFR tommorrow in Mid-Wales. My MSA for one leg is 4,300ft (over high ground of 2,907ft). If the cloudbase is similar to today it will be about 3/8 at 5,000. To stay legal / safe it would be impossible to fly quadrangles at 1013 mB. Even if it were possible to comply with the quadrangle rule and use flight levels, I would be much more comfortable flying on regional QNH so that I can equate my altimeter reading to the heights above mean sea level on my chart rather than on a theoretical pressure setting that provides no information to me of a practical nature for terrain avoidance.
Mr W.
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Mr Wolfie
The reason for so-called "full airways" equipped aircraft having 2 altimeters is that number 1 is set to 1013 in the cruise, and number 2 is set to Regional pressure setting for terrain awareness/avoidance reasons.
If flying on 1013, you need to do rather more maths during your flight planning - but that is the world of IFR ops.
The reason for so-called "full airways" equipped aircraft having 2 altimeters is that number 1 is set to 1013 in the cruise, and number 2 is set to Regional pressure setting for terrain awareness/avoidance reasons.
If flying on 1013, you need to do rather more maths during your flight planning - but that is the world of IFR ops.