Setting aerodrome QFE/QNH on arrival

[Final 3 Greens]

Perhaps my landings are so smooth that the runways just feel flat......

But on the other hand self delusion is a dangerous things

[Who has control?]

Depends on the type of landing gear, now in a bouncy Cessna which is sitting on spring steel, all landings feel rough, but in a aircraft with shock absorbers, they are all smoooooooth.


As an experimant, try settin all the altimeters in in your club aircraft to a standard value, eg 1010mb, and see what they height all read. Or try setting them to O' and see what QFE they produce. I'll bet they vary by +/- 50'

[Pronto]

Ok peoples, I put my hands up! When I trained for my licence, many moons ago, the only text books available were Branson and Birch, and Ron Campbells books. Campbell didn't make any mention of following what I've always believed to be the US practice of using QNH for landing. I've now looked at a copy of Thom which does.

However, it's not only me who didn't seem to be aware of the (UK) use of QNH for landings, see http://www.chirp.co.uk/air_transport/FB45.htm The information given in that document represents what I understood to be the UK standard practice. I cannot comment on commercial operations - I have no experience. However, since this is a Private Flying forum, I wouldn't expect to have to!

Having looked at http://www.chirp.co.uk/air_transport/FB46.htm I can only accept Bookworm's correction - it seems that I've tried to be too clever with the old Q code.

I now bow out.

Pronto

[tomahawk1673]

thanks for the replies everyone.

My instructor seemed to teach me a few things the commercial way, perhaps this is one of them. So my next question is what do commercial pilots do with regard to setting QNH and Reginonal QNH on departure? Or am I asking in the wrong place?

TIA

[AMEX]

Commercially speaking, I depart using the QNH and once cleared to a FL then use the Standard setting.

Most commercial operators use QNH only (apart from standard of course) because on Go arouund that's how the procedure should be flown. Some chose to set the QFE for the instrument approach which is perfectly acceptable... as long as they don't forget to change the setting during the usually quite busy times following the missed approach.

As for the regional QNH, well never really use it since this is good oustide controlled airspace and below transition level. IMC that low ain't my cup of tea.
Some light aircraft operators like to save a few pounds in avoiding airways charges so tend to fly low. Luckily not the way our boss sees it. How professional is it to be bumping around at low level, dodging between airspaces with PAX down the back ? ( and remember even in a Seneca they pay a fair amount of money for the privelege).

[G-SPOTs Lost]

Nope sorry chaps

QFE is the HEIGHT given above the nominated landing runway threshold. Those BPL flyers will note there is a millibar difference between QFE on RWY13 and RWY28

I also hope that you would fly around with the most accurate altimeter setting available to you at that time.

As far as JAR OPS is concerned Im pretty certain that when flying within 25 miles of a reportable QNH measuring facility then that facility will be able to offer you the most accurate altimter setting so that you can maintain accurate terrain clearnace

One of my pet hates is a student who on departure sets the regional QNH for a flight that is to remain in sight of the airfield. Regional QNH is the LOWEST forecast QNH foreseen within that hour over an area covering many thousands of square miles.

You have a responsibility to yourself and your passengers to know EXACTLY how high above msl you are.

Flight Levels also have a terrain clearance issue, normally within the UK transition level is FL30 extending I believe to FL60 within the London TMA. This is because in the UK terrain is pretty much limited to below 3000 feet. In the states where there is a much highre level of terrain diversity TL is FL180 throughout the country even thought the highest ground in Florida is a little over 300 feet

Upon leaving Majorca TL is 70 which is incidentally the MSA for that area also.

[Them thar hills]

Altimeter setting

Tom
Regional QNH v aerodrome QNH..... a minor point but if you need to remain vertically clear below SRA's etc you should use the QNH of the associated aerodrome. Regional QNH will always cause you to be higher than actual. ATC may sometimes offer the regional, if you happen to be talking to them.
Our wise CFI said don't fiddle with altimeter settings unless you really know what they should be ! And he meant it.
Commonly I'd fly on local QNH from ATIS.
And to keep winding off 40 mb before landing gets to be hard work !!
Round here the fields aren't flat, men are men and the sheep are nervous !!

Fiddle-ye-not

TTH

[Gerund]

I have been reading this thread with interest. I think the key is to be adaptable and use the setting that fits the circumstance - and to be FULLY aware of the setting one is using and why.

Sitting here in Johannesburg, I have just listened to the latest Johannesburg International ATIS. It gives QNH as 1026 and QFE as 837. If you are flying privately it is very doubtful if your aircraft will be equipped with an altimeter that can be wound to 837. So if you have come over from the UK for a flying holiday to fly in SA, you will have to use QNH.

You may wish to go on a continental, and beyond, touring holiday. If you fly into Russia, the arrivals and departures will be on QFE below transition level AND altitude respectively. The 'heights' will be given in metres based on QFE and at many, even moderately high, airfields you will not be able to wind your altimeter far enought to set QFE. You will now be faced with converting heights in metres to altitudes in feet both for landing and take off.

In Kenya, due to the scarcity of reporting stations, there are not enough QNH reports to enable pilots to determine the lowest flight level which will ensure terrain clearance - lowest flight levels are based on the application of the 'climatological method'!

Interestingly, in my Aerad Flight Information Supplement (which for just a few quid makes fascinating reading) Altimeter Setting in the UK is ICAO standard....except that Aerodrome QNH will be used at or below Transition Altitude for arrival, departure, ....... No mention of QFE.

Or try Peru - QNH within Lima CTA up to 3000 ft, elsewhere 1013.2 mbs.

As to a comment made that transition altitudes are related in some way to terrain clearance............. Maybe in some countries?, but please don't try taking off, climbing through transition altitude, setting QNE and setting sail. What is the height of Snowdon? And the Andes for that matter.

It all comes down to using the setting that fits the bill and KNOWING what setting you are using, and WHY you are using it. That way private flying will be safe and fun, WHICHEVER country you are in!

[BillieBob]

Who mentioned the Pedant Police??

Oh dear, G-SPOT really is lost this time

QFE is the HEIGHT given above the nominated landing runway threshold.

Not true! QFE is the atmospheric pressure (not height) at the aerodrome elevation (OR at runway threshold) - See UK AIP GEN 2.2. Normally, if you are given QFE it relates to the pressure at the aerodrome reference point which is defined as the highest point on the landing area. If, however, you are given 'Threshold QFE', this relates to the threshold of the relevant instrument runway - See UK AIP ENR 1.7 Para 5.2.2.1

Flight Levels also have a terrain clearance issue, normally within the UK transition level is FL30 extending I believe to FL60 within the London TMA.

Not true! The Transition Level varies with atmospheric pressure, it may be FL30 one day and FL35, or FL25 the next. The Transition Altitude however is 3000ft amsl over most of the UK, except for certain areas where it is, variously, 4000ft, 5000ft or 6000ft amsl - See UK AIP ENR 1.7 Para 4.1

In the states where there is a much highre level of terrain diversity TL is FL180 throughout the country

Not true! The terms Transition Level and Transition Altitude are not used in the USA. However, FAR 91.121 describes the altimeter setting procedure and requires that, below 18000 feet MSL (c.f. Transition Altitude) the current reported altimeter setting (c.f. QNH) is used and at or above 18000 feet MSL the altimeter subscale should be set to 29.92"Hg (c.f. Standard Pressure Setting). A table of the lowest usable flight level is also provided, which varies with - guess what - Current Altimeter Setting (i.e. atmospheric pressure).

In broad terms, therefore, the US system is identical to the UK except that the 'Transition Altitude' is set at 18000 feet amsl instead of varying between 3000 and 6000 feet amsl. In both cases, the 'Transition Level' varies with atmospheric pressure.

[distaff_beancounter]

Height/Altitude by GPS

Now that you lot have totally baffled me with more facts about altimeter settings, than I had ever thought about, may I ask another question, please?

Last Saturday, I & another PPL were trying out a Garmin GPS Map 196. This has a "panel page", which amongst other instruments, shows a conventional altimeter.

When we were flying mostly under 3000ft, using airfields' QNHs, the altimeter on the GPS gave readings that were within about 30ft of the aircraft's 2 altimeters. We travelled over a route that had actual ground level heights between about 20ft and 600ft.

I assume that a GPS measures an absolute distance between the A/c & Earth's surface, and, of course, takes no account of varying barometric pressures.

BUT, where/how does the GPS measure the Earth's surface?
Mean Sea Level?
Or actual elevation under the A/c?
Or some sort of average from a database?
OR ....... ????

[Final 3 Greens]

Distaff

At the risk of being shot down, for I am no expert, I reckon that a GPS actually works out your altitude <under> the satellites that it is triangulating from, by timing the radio signals from the satellties against an almanac database.

I am hypothesising that the GPS is set to calculate altitude from a sea level base figure.

Thus is the time for the radio signal should be x and is actually x-0.0000001 (or whatever), then the GPS calculates you are xxxx feet above sea level.

That is how I understand the horizontal triangulation to work and it seems reasonable for the vertial to be similar.

Hope this bit of saturday morning speculation helps! Anywone out there know for sure??

[G-SPOTs Lost]

Not quite so lost as you may think....

Speak to your local scare traffic controller, in the vast majority of cases ATC will give you threshold QFE without you realising. Consider Leeds Bradford where 32/14 has a 1% gradient over a 2km+ runway! giving airport QFE could be as much as 50ft out. Again its all about accurate information

Instead of swallowing the AIP and repeating it, I suggest that you look at general gist of what i was saying which was trying to encourage people to fly around on the most accurate info available, which is actually what the thread was about. Having said that you are right of course TL does alter with baro pressure, but who would want to fly at FL25??? and be wetting their pants about terrain clearnace the whole trip, that would be silly wouldn't it ????

Besides I was actually refferring to arrival not departure which incidentally is where most CFIT incidents occur.

How many US traffic controllers would climb an aircraft to sit on Transition level or altitude...answer none you are inviting an accident. More often than not when passing a lower altitude you would be cleared onto a flight level exceeding 18000 or FL180. It is good practice then to set standard so that altimter checks can be conducted in the climb. Only resetting them then should the controller "reclear" you to a lower sub18k, FL180 level.

Just exactly how many FAA ATPL's do you think use this chart you are refering to operationally on a daily basis, flying airplanes that will actually fly above 18000 feet

I would also suggest you check your weight & balance it must be questionable with all those AIP's on the back seat to refer to in flight..........

[Gerund]

distaff_beancounter, what an interesting thread you have running here.

I am afraid that I don't know the answer to your question about how the GPS calculates altitude, but presumably on some model of the earth - bearing in mind the damn thing isn't spherical! Using this model I assume the unit calculates altitude, ie height above msl.

This leads on to another interesting thing.

A previous poster said "You have a responsibility to yourself and your passengers to know EXACTLY how high above msl you are."

Well, this is down to interpretation; I am sure he didn't mean 'exactly'.

Altimeters are calibrated on the ISA which we all know doesn't exist in real life. When an altimeter is set to QNH we assume it should give us a reasonably accurate altitude for terrain clearance. But does it?

If our altimeters are calibrated on the ISA and this doesn't exist, what sort of errors are we looking at?

The last time I checked the two altimeters in a plane I was flying at FL250, and wound them both to the local QNH, they read within 50ft of each other. And the GPS figure? Around 1,500 feet difference (fortunately in the right direction).

Makes you think doesn't it!




So what about an approach to an airfield in IMC with these sort of differences? Not a problem as the airfield observes its own barometric pressure and then adjusts this using a table of corrections (based on the ISA) to give an airfield QNH. As the altimeter is calibrated on the ISA you will have accurate altitudes for the approach using your ISA calibrated altimeter.

But dig a hole down to sea level and observe the QNH there. It will be different.

Vive la difference!

[tomahawk1673]

Thanks for your help everyone, i'm not sure where my instrucctor got his numbers from, it's a shame, because he's moved away to work for an airline, and he was quite strict on them...

Scenario: If doing some local flying within 10 to 20 miles of the airfield, is it worth getting the reginal qnh if not passed and set that, along with requesting FIS?

[Gerund]

tomahawk1673 - I would advise that you set airfield QNH when you depart and leave it there until you return. Then set airfield QNH (or QFE if YOU prefer) when you contact the airfield prior to entering the circuit.

I assume you will be flying in VMC, so you can establish your own terrain clearance.

It makes sense to obtain a FIS while you fly around, but in uncontrolled airspace this is up to you. For practice, and that additional margin, why not obtain a RIS - they are so helpful and friendly.

The key thing is to have fun and enjoy the flying. Don't let all the people with their fancy theories, rhetoric, and 'must do', 'don't do' rubbish detract from the reality that flying is straightforward and enjoyable.

Yes, attention to detail is important, but if you listen too much to those who 'know', you'll miss out on the wonderful FREEDOM that flying offers!

In the immortal words of that ad, 'Just do it'.

[bookworm]

d_b

The GPS calculates the position as an X, Y, Z position in 3-D space with an arbitrary reference frame. It then compares this with an ellipsoid (egg-shape) which approximately represents the surface of the earth, to give a position and altitude.

Even neglecting measurement errors, this may differ from the altitudes or elevations we know and love because the "mean sea level" we use as a reference is not a simple ellipsoid, but rather a complex shape that depends on terrain, known as the "geoid". In principle the difference (typically no more than 100 ft) could be provided in a lookup table in a GPS receiver, but usually isn't.

+-30 ft as you saw is unusually good -- not so much because of the accuracy of the GPS but rather because of the inherent errors of altimetry!

[G-SPOTs Lost]

Gerund

Wise words indeed - my apologies

[slim_slag]

How many US traffic controllers would climb an aircraft to sit on Transition level or altitude...answer none you are inviting an accident. More often than not when passing a lower altitude you would be cleared onto a flight level exceeding 18000 or FL180.

There is nothing unusable about FL180 in the US as long as regional QNH is 29.92 or above.

[Final 3 Greens]

bookworm

Thanks for that - I thought it must be something along that line!

[distaff_beancounter]

Final 3 Greens, Gerund & bookworm Thanks very much for your interesting & informative replies.

After posting my question, I realised that I was being a bit of a bozo (or technophobe) & that the base line for GPS's calculating altitude, had to be some arbitrary calculation for mean sea level.

The main thing that I have learnt from this thread, is just how many limitations & variables, & hence errors, that are just a fact of life in altimetry.

So, when in IMC, never, never take a chance on going any lower the the MSA, or MDH+50' for altimeter error, when on an instrument approach.

That is, unless your A/c has a Radar altimeter.
Now ..... I wonder how one of those works.