The good old altimeter problems

[WhiteKnight]

I came across this question:

A helicopter pilot starts from his base at 3600ft heliport elevation. The QNH is set to the local value 1032 hPa. The temperatures are 15°C above ISA. What is the approximate altimeter reading upon landing in a mountain area with true altitude 8050ft, assuming unchanged altimeter setting?

8050
8300
7800
7600

answer is c)

Please correct me if I`m wrong but I think with a correct altimeter setting, and standing on an airport the altimeter will always show the correct elevation regardless of my actual temperature. Is this right?If yes, my altimeter reading should tell me the correct elevation If I land at another airport assuming the QNH has not changed. Am I right? So the answer should be a)

[Vee One...Rotate]

I'd think (a) as well. If true alt of starting point is 3600 MSL and true alt of end point is 8050 MSL and a QNH of 1032 gives you 3600 indicated at the start point then it should indicate 8050 at the end point...all other things remaining equal.

Surely???

At least if the WhiteKnight has the question exactly right.

V1R

P.S. I speak as a PPLer with about 5 hours logged...!

[411A]

If the same altimeter setting (QNH) has received from both stations, the altitude indicated would be correct, assuming no instrument error.

In the USA anyway, the local altimeter setting (QNH) is corrected for non-standard temperature, at the particular station location.

I think.
Now, not having taken one of these exams in over thirty years...maybe things have changed in the altitude indication department.:uhoh:

[oxford blue]

The formula is:

True alt = Indicated alt + (ISA dev x 4/1000 x ind alt)

Strictly speaking, it's not quite 4/1000, it's about 3.6/1000, but that's close enough. If you you want the full theory, it's in the Oxford Gen Nav notes, which fully cover the principles of slide-rule type nav computers.

However, the whole point of QNH is that your altimeter reads the right value when you touch down at the airport, whatever the actual temperature.

At the airport, they don't know what the sea level pressure is - unless there a convenient mineshaft nearby that goes down there.

So they measure the QFE. But they know that your altimeter won't correct for temperature error. So when they reduce it to sea level (QNH) they use the standard ISA atmosphere, whatever the real temperature.

That way, your uncorrected altimeter will give the airport altitude on touchdown with QNH set.

Nevertheless, your altimeter is still affected by temperature error. But only, now, on the uncorrected bit. That's the bit between you and the touchdown elevation.

In this case, you are (8050-3600) 4450 feet above touchdown.

Now, treat 4450 as the true alt , and work out all the calculations from a datum of 3600, where the altimeter reads correctly.

As I said,

True alt = Indicated alt + (ISA dev x 4/1000 x ind alt)

4450 = Ind Alt + (+15 x 4/1000 x Ind Alt)

4450 = Ind Alt + (0.06 Ind Alt)

4450 = Ind Alt x 1.06

Ind Alt = 4450/1.06

Ind Alt = 4198

Now remember, that's only the difference from 3600. So add 3600 to it.

4198 + 3600 = 7798

Seems pretty close to answer (c) to me.



However, WhiteKnight, I have another point to make. You recently posted something on Polar Stereographic charts. I suspect that the question you posted was wrong. Nevertheless, this forum is not 'wannabes', which caters for JAA questions, but 'Tech Log', which is much broader. Test pilots, flight test engineers, systems designers, and hugely experienced academics and training pilots contribute here - often, as in your case, to queries which are just plain wrong.

You could at least acknowledge their efforts and say "Thank you" on the forum afterwards. You didn't after your last post.

Try and be a bit more gracious and gentlemanly in future. It doesn't cost much to say thank you - especially when it's you who made the work by getting the detail wrong. It's a lesson for life - not just pprune - if you want to make a success of this profession.

And post JAA questions in 'wannabes' in future. Save this forum for the highly technical stuff.

[Max Angle]

Save this forum for the highly technical stuff. Well that little lot was technical enough for me and I've been at it years!.

[oxford blue]

That, I guess, was a standard JAA question.

If you want to say that it doesn't have much in common with normal practical piloting for a living, I would agree with you.

Buit let me tell you, as an experienced ground school instructor, I recognise the dead hand of the JAA (or an FTO preparing students for the JAA).

After all, no real pilot would ask such an unrealistic question, would they?

I'm not saying that you don't have to be aware of temperature error. But most real pilots are far more concerned about cold temperatures, not warm ones.

[Old Smokey]

WhiteKnight,

Altimeters are calibrated for ISA conditions, and are subject to local Pressure and Temperature deviations from ISA.

With Temperature remaining at ISA, but Pressure varying, it can be simplisticly viewed as the entire atmosphere moving up or down in relation to Sea Level. The pilot can correct for this by adjusting the Altimeter sub-scale to the local QNH (1032 in your example). If the temperature continued to remain at ISA, Indicated Altitude would then be correct at all Altitudes.

If Temperature varies from ISA, it can be simplisticly viewed as the entire atmosphere vertically expanding or shrinking, and the true vertical difference between pressure levels similarly expanding or shrinking. In temperatures above ISA (your example) the atmosphere has expanded, and you will have more actual altitude than indicated (safe), in colder than ISA temperatures, you will have less actual altitude than indicated (unsafe).

In your example, the QNH set at the 3600 ft AMSL departure point has compensated not only for the Pressure variation from standard, but the Temperature deviation from standard (It's now not actually QNH but QFF). This was possible because the actual elevation was known and may be set on the altimeter. As you now climb away from your departure point, the Indicated Altitude is in error in direct proportion to the ratio of the Mean Temperature between yourself and the departure point, and the Mean ISA Temperature between yourself and the departure point, in degrees absolute of course. Here's the Numbers -

Dep = 3600 ft @ QNH 1032 = 3092 Pressure Ht.
Arr = 8050 ft @ QNH 1032 = 7542 Pressure Ht.
Mean Pressure Height = 5317 ft : ISA Temp = 4.466°C = 277.616°A
Actual Mean Temp at ISA + 15°C = 19.466°C = 292.616°A

INDICATED difference in Altitude will then be the Actual Elevation difference (8050 - 3600 = 4450) multiplied by the ISA Mean Temperature, divided by the Actual Mean Temperature. i.e. -

4450 X 277.616 / 292.616 = 4221.9 ft

Actual Indicated Altitude will be 3600+4221.9 = 7821.9 feet.

This is obviously SAFE, if you climbed in cloud to 8050 feet indicated, you would be 228.1 feet ABOVE ground. The opposite is true in colder than ISA conditions, where you have LESS actual Altitude than indicated, an UNSAFE situation.

Upon arrival at your 8050 ft landing point, you would find that you would have to reset your Altimeter to 1040.3 hPa to indicate correct altitude there.

This question MIGHT have come from a Wannabe, but I don't think that this is a Wannabe question, it should stay in the Technical area. Some other posts here suggest that there's still some experienced aviators 'out there' with some Altimetry misconceptions.

Flying from an area of low pressures and/or temperatures to an area of higher pressures and/or temperatures is safe, but NEVER forget the opposite case - FROM HIGH TO LOW, LOOK OUT BELOW !

Old Smokey

[enicalyth]

Hi Guys!
I have been blasted twice for not RTFQ but isn't this, well, pretty basic? As pilots it is thumped into us in groundschool that for each 10degC the OAT is warmer than ISA increase the indicated altitude by 4% to give true altitude and for each 10degC the OAT is cooler than ISA decrease the indicated altitude by 4%. So if the airfield is 3600 ft and the mountain is 8050ft and also +15 ISA the helo pilot has moved 4450 ft upwards but the QNH altimeter has moved only 4200ft and thus "should" read 7800ft. All assuming we can treat humid air as a dry ideal gas. (Which we don't in meteorology or propulsion but it'll do for now and I won't bother with the regs for what constitutes a serviceable altimeter and what it "could" as opposed to "should" read). So my answer is c) and I am pretty worried if someone disagrees with me because one of us is badly mistaken and I am pretty sure it isn't me. (What is being thumped into people at groundschool?) My next worry is more technical. I was taught at school (not Uni) that for a dry ideal gas V/T is a constant provided that pressure stays the same (Charles' Law)and PV is a constant provided that temperature stays the same (Boyle's Law). Thus for a dry ideal gas P1V1/T1 = P2V2/T2. That pretty well wraps up the whole she-bang. Is it just not fashionable to teach this anymore either?
Sorry if I sound a bit abrupt this evening.

PS Mrs E has asked me to apologise before tea so I do. Also Old Smokey, if it is say 30 feet with every hectopascal and +/- 2 hectopascals tidal variation from the mean in Oz that could be 120ft in six hours - twixt low and high tides. If I park my Piper near a beach in the afternoon and come back for it later that night do I have to dig the altimeter out of the sand?

Chiz from the chastened enicalyth

[WhiteKnight]

@ oxford blue

Thank you for your detailed explanation, that helped me a lot.
I apologize for not expressing my thanks on the other thread. I will not pester this forum again with simple JAA questions. Nevertheless, I don`t think the question was wrong, I still try to grasp the calculation.

@ all the others

Many thanks for your comments I fully understand now.

[Old Smokey]

Gee whiz enicalyth, I hadn't thought about that one. Can I have some time to think about it, like, maybe a month or two.:O

Cheers Mr. Water Vapour,

Old Smokey

[enicalyth]

Carn Old Smokey do I dig down or do I get out the steps? Mrs E thinx I am nuts and probably you are too.

[bookworm]

If yes, my altimeter reading should tell me the correct elevation If I land at another airport assuming the QNH has not changed.

I think the key concept here is that a QNH is not just specific to a 2D location but also to a level in that 2D location. Thus the QNH at a heliport at 8050 ft perched on stilts above the one at 3600 ft will not be 1032 hPa (unless the temperature profile happens to be ISA).

[enicalyth]

Smokes!

Being a technophobe I shall probably screw up this jpeg but it purports to show yestiddy at YSSY. Dew Point (heh, heh); barometer; temperature. [One day gorgeous, perfect the next]. Altimeter reads 20ft on parking 1020hPa QNH; I return from the beach, 1016QNH and if the jpeg doesn't work assume constant temperature ('cos we can!). Mine is a sypathetic altimeter so do I dig down for it or fetch tall steps. Mebbe that might help the young 'uns but I do believe that in this part of the world, for QFF/QNH freaks the semi-diurnal tidal variation is 120ft or 4hPa.

How is SIN PER these days? Took my daughter to Rottnest as she hadn't seen a quokka. Thazzands of 'em at the airport. Must be trying to get jobs on the mainland.e:/YSSY wunderground

[Milt]

A bit off thread but relevant.

Clock Type Altimeters.

What idiot instrument designer inflicted those intruments on we uncomplaining aviators. Uncomplaining for a long time but with little result until tape instrumentation.

Never in thousands of hours scanning the damn thing could I subconsciously interpret without that extra little bit of mental calculation that the movement of the needle on the right of the dial indicated that the aircraft was actually going in the reverse sense to the needle movement and at odds with the VSI..

Has anyone ever been able to completely retrain the brain to accept this anomoly ?

[411A]

Really, Milt?

Difficult to read?
Now, I will admit that the older three pointer types are subject to mis-reading from time to time, but the drum and pointer types took care of this rather nicely, I thought.

[White Knight]

411a - drum and pointers still not very helpful, even if you've got your bifocals on:cool:

WK - why have you copied my user name:mad: :} :yuk: :ugh: :\