Meteorology question

[Rodent1982]

Hi folks,

Came across this question in the Oxford CBT Meteorology mock test.

An aircraft flies aerodrome "A", where QNH is given as 1020mb, to aerodrome "B", where the QNH is given as 999mb. Aerodrome "A" is 800ft above mean sea level and aerodrome "B" is 500ft above mean sea level. If the altimeter sub scale is not changed from 1020, what is the altimeter indication on landing. (ASSUME 1mb = 30ft).

a) 1430ft.
b) 130ft.
c) 1130ft.
d) -130ft.


Now, I might just be tired and am missing something here, but surely, none of the answers are correct. By my calculation, the original QNH is 1020, and the new QNH is 999 (drastic change, but hey) meaning a 630ft change. And as it's been calibrated for 1020, i'd say on landing the altimeter should read 630ft?

The answer is c) 1130ft. But surely this would indicate a mistake in the question, and the QNH for aerodrome B is actually the QFE?

[AlphaMale]

I'm with Airbus :ok: ... You did the calculation right but forgot to add the field elevation of 500ft back onto the 630ft.

I'm sure if you left it blank in an exam and came back to it at the end it would click ;-)

Good luck.

[Rodent1982]

Yes, makes perfect sense now... brain momentarily fried... :ugh:

[IO540]

One of those wonderful trick questions which I have never ever ever not even once ever had to work out in any real flying situation.

Why?

Because, when flying for real, one flies on some assigned QNH, or a flight level, and one sets the altimeter to the former, or to 1013.

The kind of mental aerobatics this Q requires is never done for real, and thankfully not because if obstacle clearance relied on getting it right, there would be a lot of wreckage everywhere.

[BEagle]

No, IO540.

It points out the hazards of failing to reset the QNH and how the altimeter will read; even though the elevation of the aerodrome of arrival is 300 ft lower than that of the departure aerodrome, the altimeter reading will over read considerably.

The distracted pilot who leaves the original QNH set and then decends to what he thinks is circuit altitude (say 1500ft) would be in for a nasty shock.

These types of questions do have relevance; easiest way to work them out is always to draw a small diagram.

Mind you, a 21 mb QNH change during the course of a typical VFR PPL spamcan flight would mean some pretty nasty weather!

[IO540]

I think anybody who knows 30ft is roughly equal to 1mb will realise the danger of setting the wrong QNH - for both obstacle clearance and busting airspace from underneath.

[S-Works]

I think anybody who knows 30ft is roughly equal to 1mb will realise the danger of setting the wrong QNH - for both obstacle clearance and busting airspace from underneath.

Quite right. But pray tell how you test to verify they knew this........

[stickandrudderman]

I tend to approach at tree-top level, adjust my altimeter to read 30ft. then climb at full throttle to circiut height over the nearest village. Works for me!:rolleyes:

[Crash one]

I approach on Regional Pressure & eyeball the rest, Land at airfield elevation. Unless it's CAS of course!!

[englishal]

The distracted pilot who leaves the original QNH set and then decends to what he thinks is circuit altitude (say 1500ft) would be in for a nasty shock.
I'm not convinced. When I fly to an airfield and have no local pressure setting, I look out of the window. If the trees look close, bad ;)...Seriously though who uses the altimeter when landing? I use it to get setup at circuit height but if it wasn't there I wouldn't plummet into the ground just because...

I would actually like to suggest that I could safely fly across country VFR with no altimeter at all (assuming no airspace and nice weather) and still manage to take off, cruise and land with no drama...

[PompeyPaul]

who uses the altimeter when landing?I like to have a quick look at it, in the PA28 as I turn base to final. It should read around 650ft AAL and it just gives a bit of comfort to see it at that as I turn final.

Although, when doing the night rating, the instruments were a total distraction. I flew better night circuits when the instructor killed the lighting inside then when I had all of the instruments. It's amazing, even with my lack of experience, how your ears have picked up what the engine should sound like rather than relying on instruments. Even a couple of times around the circuit the lights were put on briefly and yet everything was set up and looked where it should be, power RPM, height etc etc

So, yeah you don't need the altimeter whilst landing, but I do like to give it a quick glance at that one point.

[IO540]

Surely one just sets the altimeter to the GPS altitude?

A GPS with EGNOS reception is much more accurate than any barometric altimeter.

The above is less of a windup than many will think! And I don't need a lesson on why baro altimetry is used.

Let's say I am flying an approach in IMC. I check the altimeter (set to the issued QNH) against the Garmin 496 altitude (one of the four screen-corner parameters can be configured to be the GPS altitude) and if the two were to disagree by say 500ft then I would go somewhere with an ILS.

Never had them disagree by more than about 20ft, at low level. At higher levels, say 10,000ft, they can disagree massively (a few hundred feet) but that is because the altimeter is wrong, due to a nonstandard atmosphere.

[englishal]

What is EGNOS?

GPS altitude is based on the gravitational equipotential of the earth (Geoid or Spheroid or something like that, I can never remember :O) - essentially using a mathematical model of the gravity of the earth as the "surface". But we all know that gravity changes around the world - for example over everest it is slightly higher due to the more mass below, and I guess over the marianas trench it is sloghtly lower. So this model is an average. Hence in some places the model may be above sea level and in others below sea level. This is why GPS altitude doesn't always agree with barometric altitude.

At least something like that ;)

[IO540]

In this part of the world, the error is of the order of 50ft - much less than most will think.

EGNOS is the European version of U.S. WAAS. It corrects these and other errors.

Also any half decent GPS contains a table of the geoid variation around the world.

On the whole, GPS altitude is far more accurate than baro altitude.

But some cheap GPSs are poor; for example the SIRF 1 and 2 chips had a permanent 200ft error. I think the SIRF 3 chip fixes that. Most cheap GPSs use one or another of these chips. However, one wouldn't normally fly with these. A Garmin 296/496 does it properly, as does any IFR panel mount unit.

[Fuji Abound]

Must be getting old - no one along to tell IO540 that he shouldnt rely on GPS - has the world gone soft, or have all those old bold pilots hung up their head sets?

Where is DFC when you need him?

[IO540]

Maybe, with 8k posts and 6 years, I have worn out their will to live :)

[cats_five]

8,500 posts in 6 years. That's 1,400 per year (approx), 3.88 per day discounting 29th Febs which would reduce it ever so slightly. That is some posting rate to keep up for 6 years...

[IO540]

Well, I do say to my kids that I don't know everything but I do have an opinion on everything :)

The age at which they first find that funny is quite interesting.

[cats_five]

PS Just to get vaguely back on topic, glider pilots are expected to show they can do a satisfactory circuit (or two) with the altimeter covered before going solo. Field landings are such a normal part of XC gliding (for those of us without sustainers / turbos) that it is vital to be able to do a satisfactory circuit into a small field of unknown height.