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QNH or QFE ?

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Old 9th Aug 2018, 09:10
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I'm a strip flier. If I'm staying local I set my altimeter to zero before departure. If I'm not staying local, I set it to field elevation. Should I change my ways before something bad happens?
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Old 9th Aug 2018, 09:14
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RPS is totally different from a QNH in that it's the lowest FORECAST pressure setting in a particular region rather than an actual measured and reported pressure setting at an airfield or other defined point.
Originally Posted by eckhard View Post
And useless for most pilots as a result.
I don't agree. We measure altitude (or height) in an aircraft not just so that we can impress our passengers about how high we actually are - although from the PA announcements in a commercial airliner you might think that that is still the case.

We measure altitude or height for at least three specific purposes:
1. To measure how high we are above an airfield or runway. This is important for circuit altitude/height, and for IFR procedures to determine when to go around or land (DA, MDA).
2. To measure how high we are related to surrounding terrain, and to achieve whatever obstacle clearance we think is necessary.
3. To have a common datum when aircraft are reporting altitude, so that two aircraft that are reported to be in the same position, but at a different altitude, are indeed a certain amount of feet apart. In other words, for collision avoidance.

RPS is useless for 1, as it is not sufficiently accurate. But it is perfectly usable for 2 and 3.
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Old 9th Aug 2018, 09:34
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One problem with using RPS in the UK is when flying below a CTA where the base is notified as an altitude; the RPS can be low enough to cause the pilot to infringe if they try to fly just below the CTA.
I once watched a stream of Hercules not talking to Farnborough or Gatwick crossing MID VOR south eastbound all indicating 2,700ft where the base is 2,500ft (this was in the days before the RAF recognised the 'rule' about using an actual QNH when flying below a TMA) because they were using the Chatham RPS. Particularly annoying for Gatwick as they were landlng on easterlies at the time.
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Old 9th Aug 2018, 09:44
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Not too long from now all primary altimetry will be GPS derived and barometric altimeter setting will be retained purely as back-up. On commercial aircraft local QNH will be auto-uploaded by data link and inputted to back-up barometric altimeters, both to confirm GPS altitude and to give redundancy. For GA pilots the option will be available to adjust indicated altitude to height for local flying.

It could be that the term 'flight level' is retained, meaning - as now - altimeter indication (in feet) divided by 100 and the word 'feet' will become redundant, removing the complications of parallel Imperial/metric systems. Perhaps for low viz ops decimal FLs for Decision Altitudes could be specified.
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Old 9th Aug 2018, 10:37
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Originally Posted by chevvron View Post
One problem with using RPS in the UK is when flying below a CTA where the base is notified as an altitude; the RPS can be low enough to cause the pilot to infringe if they try to fly just below the CTA.
I once watched a stream of Hercules not talking to Farnborough or Gatwick crossing MID VOR south eastbound all indicating 2,700ft where the base is 2,500ft (this was in the days before the RAF recognised the 'rule' about using an actual QNH when flying below a TMA) because they were using the Chatham RPS. Particularly annoying for Gatwick as they were landlng on easterlies at the time.
Good point. More in general, we all tend to report a number which we think is our altitude (or height), but omit to include the information how that altitude or height was derived. Heck, we even omit the word "height" or "altitude" on most occasions.

I was doing my IMC rating at Leicester (almost 500' elevation), when QFE was still used a lot for VFR flying. So I was entering the practice hold (situated more or less overhead) at 2000' on the QNH (and reporting "holding at 2000") while other aircraft were doing VFR overhead joins at 1500' on the QFE (and reporting "overhead join at 1500"). It did not take very long to realise that we were actually just feet apart. From that moment on I started reporting my altitude as "2000 feet on the QNH, 1500 feet on the QFE".

Mixing two altimeter datums in the same airspace is dangerous. For safety, all aircraft in the same airspace should use the same datum. Whatever that datum is, I don't care, as long as everybody knows and uses it.
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Old 9th Aug 2018, 10:54
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Originally Posted by Discorde View Post
Not too long from now all primary altimetry will be GPS derived and barometric altimeter setting will be retained purely as back-up. On commercial aircraft local QNH will be auto-uploaded by data link and inputted to back-up barometric altimeters, both to confirm GPS altitude and to give redundancy. For GA pilots the option will be available to adjust indicated altitude to height for local flying.
I don't think so. Not because it's not possible, but because there needs to be a transition period to get from one system to the other. And during that transition period, some aircraft will base their altitude measurement on a GPS reading, and others on a barometric reading. However, there is no way to convert an accurate barometric-based altitude reading into the equivalent GPS-based altitude reading and vice versa unless you know not only the local QNH, but also the exact environmental lapse rate at your given position. Heck, you even need to know the actual humidity across the whole column of air below you. Using the standard ISA lapse rate for this instead may lead to hundreds of feet of error if you're flying at FL300. That's a whole order of magnitude worse than the normal accuracy of analog, mechanical barometric altimeters, and even more orders of magnitude worse than GPS-derived altitude. So during the transition period we would need to use significantly more separation than what's applied now. While the current trend is to reduce separation (RVSM) to improve airspace capacity. I don't think that would work.

Even transitioning from 25 kHz-spaced VHF frequencies to 8.33 kHz spacing will eventually take something like 10 years. And that's a transition that's backwards compatible (8.33 kHz radios can be used in 25 kHz airspace).
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Old 9th Aug 2018, 11:20
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Originally Posted by BackPacker View Post
However, there is no way to convert an accurate barometric-based altitude reading into the equivalent GPS-based altitude reading and vice versa unless you know not only the local QNH, but also the exact environmental lapse rate at your given position. Heck, you even need to know the actual humidity across the whole column of air below you. Using the standard ISA lapse rate for this instead may lead to hundreds of feet of error if you're flying at FL300.
Adjusting GPS altitude to match indicated barometric altitude would be accomplished electronically within the apparatus without pilot input required.
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Old 9th Aug 2018, 11:26
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Originally Posted by Discorde View Post
Adjusting GPS altitude to match indicated barometric altitude would be accomplished electronically within the apparatus without pilot input required.
And just how long and how much money would it take for the CAA to certify and approve such an electronic system?
You can go to a shop today and buy a Casio wristwatch which will display your altitude, but I don't think the CAA would accept that in lieu of a barometric altimeter.
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Old 9th Aug 2018, 11:35
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Originally Posted by Discorde View Post
Adjusting GPS altitude to match indicated barometric altitude would be accomplished electronically within the apparatus without pilot input required.
And where would this gadget get its input data from? As I said, the gadget needs the local QNH and the actual local environmental lapse rate (+humidity) of the whole column of air below you, to do the calculation. Otherwise your electronic gadget is just going to be a GIGO system (Garbage In - Garbage Out).
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Old 9th Aug 2018, 11:44
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Originally Posted by BackPacker View Post
As I said, the gadget needs the local QNH
And if you need and therefore have got that, why not just display it?

Electronics are not inherently better; sometimes simple is the way to go, using the environment rather than man-made infrastructure.
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Old 9th Aug 2018, 12:03
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Originally Posted by BackPacker View Post
And where would this gadget get its input data from?
From the aircraft's static ports.

The point about GPS derived altimetry is that it removes the need for altimeter setting, with its attendant drawbacks. Every time a subscale setting change is required there is the risk of mis-setting or omission of resetting. In European airspace, with low transition altitudes, the authorities are obliged to publish notams every time a deep low pressure weather system passes through, warning pilots about the dangers arising from failure to reset when there are large QNH/1013 differences.

The transition process will be similar to the gradual adoption of RVSM and RNAV approaches, with the procedures introduced initially into low traffic density airspace to prove efficacy.

Last edited by Discorde; 9th Aug 2018 at 12:58. Reason: Re-write
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Old 9th Aug 2018, 18:51
  #52 (permalink)  
 
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Originally Posted by BackPacker
Mixing two altimeter datums in the same airspace is dangerous. For safety, all aircraft in the same airspace should use the same datum. Whatever that datum is, I don't care, as long as everybody knows and uses it.
As an ex-controller who used to work terminal areas, having more than one level datum was just day-to-day business. Aircraft coming into my area of responsibility were often at or descending to a FL, below TA QNH was the common pressure setting used and around the aerodromes QFE might well have been used. Include in the mix aircraft transiting the control area which might have been on the Regional Pressure setting, with the added bonus that being close to the boundary between two Altimeter Setting Regions, the RPS might have been different depending on whether the aircraft was coming from the North or South.

The rules of the game were to separate aircraft that needed separating and give traffic info on those which didn't. For separation there was a simpler system that displayed what FLs were separated from aircraft below TA and for traffic info, levels rounded to the nearest hundred feet were easy to to work out (and if they were VFR and not going to be in my airspace for long I saw no reason to get all aircraft to change to one of the aerodrome QNHs). I don't dispute that more than one pressure datum means that there is more to think about, but it concerns me to think that there may be pilots or controllers who have trouble with simple altimetry calculations when values rounded to the nearest hundred feet will suffice for most purposes.
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Old 9th Aug 2018, 21:18
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Flying vfr east east from Inverness, I'm passed to Lossie Radar (Air Force) who tell me to fly on Lossie QFE ###. I'm remaining outside the MATZ, but in the Area of Intense Airial Activity.

Last edited by Maoraigh1; 9th Aug 2018 at 21:22. Reason: Add
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Old 10th Aug 2018, 11:48
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Quote... I'm a strip flier. If I'm staying local I set my altimeter to zero before departure. If I'm not staying local, I set it to field elevation. Should I change my ways before something bad happens?
Really you should not twiddle with the knob on the front of the altimeter, the manufacturer will have set it correctly, and it should not be altered, unless you are an Instrument Technician.
Also there is no real good reason to report your altitude to ATC, as they have your Transponder reading on their displays, and their system will alert them of any clashes.
.
Problem solved....
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Old 10th Aug 2018, 16:26
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scifi
Assuming you have a transponder with Mode C which is within tolerance. ATC won't know if it's within tolerance unless you speak to them and they verify your altitude; then "problem solved".
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Old 10th Aug 2018, 16:28
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Originally Posted by scifi View Post
Quote... I'm a strip flier. If I'm staying local I set my altimeter to zero before departure. If I'm not staying local, I set it to field elevation. Should I change my ways before something bad happens?
Really you should not twiddle with the knob on the front of the altimeter, the manufacturer will have set it correctly, and it should not be altered, unless you are an Instrument Technician.
Also there is no real good reason to report your altitude to ATC, as they have your Transponder reading on their displays, and their system will alert them of any clashes.
.
Problem solved....
If ATC identify you using SSR, they have to validate any Mode C reading associated with your transponder code which requires you to report what you are seeing on your altimeter.
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Old 10th Aug 2018, 17:01
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In which case one pushes the appropriate button on the transponder, and reports what the display shows... Problem solved, indeed.
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Old 10th Aug 2018, 17:50
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Originally Posted by Jan Olieslagers View Post
In which case one pushes the appropriate button on the transponder, and reports what the display shows... Problem solved, indeed.
No.
The altitude encoder transmits a reading based on 1013.2 hpa which is then converted by the radar display on the ground which has the current QNH fed into it and updated when necessary.

Last edited by chevvron; 11th Aug 2018 at 14:37.
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Old 10th Aug 2018, 23:02
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Originally Posted by LookingForAJob View Post
As an ex-controller who used to work terminal areas, having more than one level datum was just day-to-day business. Aircraft coming into my area of responsibility were often at or descending to a FL, below TA QNH was the common pressure setting used and around the aerodromes QFE might well have been used. Include in the mix aircraft transiting the control area which might have been on the Regional Pressure setting, with the added bonus that being close to the boundary between two Altimeter Setting Regions, the RPS might have been different depending on whether the aircraft was coming from the North or South.

The rules of the game were to separate aircraft that needed separating and give traffic info on those which didn't. For separation there was a simpler system that displayed what FLs were separated from aircraft below TA and for traffic info, levels rounded to the nearest hundred feet were easy to work out (and if they were VFR and not going to be in my airspace for long I saw no reason to get all aircraft to change to one of the aerodrome QNHs). I don't dispute that more than one pressure datum means that there is more to think about, but it concerns me to think that there may be pilots or controllers who have trouble with simple altimetry calculations when values rounded to the nearest hundred feet will suffice for most purposes.
That is amazing. I'm genuinely glad none of the controllers I deal with are faced with that game, given my base in one of world's densest packed terminal areas (and all traffic on the same altimeter setting). The situation described in one of those classic British situations where a totally inefficient non-system is made to work through the application of skill... and those concerned think it is normal!

Last edited by Silvaire1; 10th Aug 2018 at 23:26.
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Old 11th Aug 2018, 14:39
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Originally Posted by chevvron View Post
No.
The altitude encoder transmits a reading based on 1013.2 hpa which is then converted by the radar display on the ground which has the current QNH fed into it and updated when necessary.
It is a requirement that the controller MUST cross check what you see on your altimeter with what is displayed on the SSR label; a +/- 200ft difference is allowed.
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