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.

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.

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.

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.

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).

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.

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).

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.

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.

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.

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....

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".

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.

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.

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!

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.

It is interesting to note in the 1949 Air Ministry book of Q Codes that QNH did not exist at that time! The two pressures used were QFE, surface pressure at an aerodrome and QFF, pressure at mean sea level. QNH first appeared in the eraly 1950s following an amendment to the method of determing the Sea Level Pressure when not at sea level.

Altimeter index
 

Is it true that western altimeters don’t have index mark which you set to barometric (pressure altitude) of runway airfield, so that on touchdown you read 0 height (QFE), especially in mountainous areas.

The last bastion of QFE is the RAF , this is largely because it helps those flying a high workload fast jet that has minimal navigation kit recover using PAR ( usually with less than ten minutes fuel remaining) Did not need another thing to think about so having the runway at zero on the altimeter was a good idea.

Things have moved on and the fast jets carry far more navigation kit now so recovering aircraft in IMC is much less fraught so once the “When I was on Lightning’s” brigade has retired you can expect a change to a more international way of doing things.

I expect QFE to continue in UK flying clubs for some time as most of them still teach ancient techniques such as Gypsy style engine management when For the last forty years they have been operating Lycomings.

Discorde: Until you can persuade flying schools to stop teaching QFE procedures then nothing will change! I have even noticed going into my nearest busy GA field that the A/G operators will refuse to give QNH to arrivals even when requested - only give it for departures!! Go figure.

You might very soon run out of scale. The one I'm using most has 940hPa as smallest value.

Might well be the same level of "intelligencies" that voted "pro" Br_x_t.

(and by the way, thanks for correctly naming A/G operators - there are those who want to call them controllers, even if they do not control any airspace)

My home airfield is 73ft AMSL thus only 2mb difference - an easy bit of maths if only QFE offered.

ATC, AFIS and A/G should all pass QNH as standard with QFE available only on request according to CAA guidance issued about 15 years ago.

chevvron: thanks for that info - good luck with pointing that out to Headcorn A/G!

I sometimes fly in Colorado and with the airfield elevation at 6870 ft there is no way you'll use QFE.

Presumably you read my bit about Headcorn on the 'other' forum, but I'm told the A/G operator when that occured has left there now.

Since the last part of flying is done visually, is it really that relevant having your altimeter show 0 when landing at all? Once you pass the DH or DA you look out the windows anyway... or am I doing something wrong here?

I guess that most of the anti-QFE contributors don't have display authorisations. The vertical limits are all expressed in heights in "CAP 1724 Display Standards Document". Converting altitudes to heights several times in an aerobatic figure is probably not good for a display pilot's health. Yes, the need to set QFE is not necessary for most regions of flight - but for some activities it makes more sense than QNH. I'm sure contributors can find other examples where QFE is the safest option. Not all flights are take off - cruise - land.

This argument has gone on for as long as I can remember. to the extent its almost pointless. I would hope that anyone performing low level aerobatics will have it firmly implanted in their minds the altitudes or heights that must be achieved both at the top and at the bottom of a manoeuvre. The QNH pilot listens in a wonder of disbelief that anyone would mess around with the altimeter at critical moments (resetting to QFE). Knowing the elevations is part of everyday flying for QNH pilots. The only rule that I demand is that you fly one or the other and then use it at all times, but never mix it. If an a/g operator refuses to pass the QFE or other wise QNH ( a FISO or ATC will never refuse) this will require a very stern chat with the fool after landing such that he/she will never refuse to do so again.

In the majority of countries throughout the world QNH is the norm.

I was born and bred with QFE & used QFE for 33 years of RAF flying.
I much prefer QNH.
(& in my glider always have height + altitude + FL all on display in front of me; modern electronics)

Discorde,

The same way as everyone else - they use the current Altimeter Setting (QNH* to you) and then convert all their gate heights to the appropriate altitude.

Of course, sometimes pilots get it wrong:


https://cimg8.ibsrv.net/gimg/pprune....598fbff4c.jpeg


Just out of interest, I checked the Winter altimeters in my club’s two-week old ASK21B. With the field elevation of 3700’ set, the sub-scale read 1020 mb. I then made 43 twists of the knob to bring the altitude to 0’ and the sub-scale read 997 mb. I was surprised I was able to do this. Last time I tried this many years ago on an old US altimeter, I ran out of sub-scale.

The observant will notice I used mb rather than inches. This is because our glider was delivered with European altimeters! We shall be having words with the factory!

Using the Altimeter Setting doesn’t seem to cause any problem for new pilots. Our students have no difficulty figuring out that the downwind leg starts at 4500’ (800’ AGL).

* I would guess that 99% of North American light plane pilots would have no idea what QNH, QFE or any of the Q codes are.

Those damn europeans!

On a side note, when I flew glider, we had a polish model, and the altimeter was in metres.... which is probably "meters" now that the UK has left the EU.

What is the "QNH pilot"? Is that someone who never flies above transition altitude?

Since this thread seems to have risen from the dead, may I ask what altimeter settings do people who fly glass cockpits with a standby steam gauge use on each? I flew with someone recently who kept QNH on the G1000 but 1013 HPa on the standby dial. I don't really see the logic of that, is there a 'best practice' for this situation?

Doesn't make sense, since the standby is a standby, and you should be able to check towards it to spot malfunction?

But I guess the more professional know the answer better than I..

India Four Two: are you sure the lower setting was 997 mb. since 23 mb. Difference does not equate to 3,700 ft?

There are some justifiable reasons for setting standard pressure on the second altimeter: should your flight altitude be close to the base of an airway/controlled airspace, which is designated as a flight level, because it could act as a valuable heads up. Similarly when flying using regional pressure (UK) then having the aerodrome QNH set on the second altimeter does a similar job when flying below the base of controlled airspace which is designated as an altitude. For those who land using QFE then having the aerodrome QNH set on the second altimeter is a common practice, ready for a go around and diversion.

But, once again we return to the possible folly, in the minds of QNH people, of fiddling about with altimeters at a critical point in the flight. QNH, QNH and only QNH is the normal standard of IFR pilots and also for those who regularly fly internationally. Outside of the UK and France QFE is rarely found. It was a common practice for commercial transport flights to land at one place using QNH and another using QFE, whatever was provided locally. This could happen many times on the same day and was a recipe for disaster. I can remember a number of tragedies with a considerable loss of life owing to the miss-setting of the altimeter by the pilots during the approach to land.