Stuck!

[DaveD]

Hmmm interesting.. Well atleast I got the concept down. It's strange though, there's a few questions like this on the website, I'll check my confuser tonight, they tend to be right, and they have reasons why too..

[Islander2]

It's strange though

Don't go wavering now, Dave, you've got it!

Pressure altitude is, by definition, what the altimeter reads when 1013mb is set on the subscale.

And we're told the altimeter reads zero when the subscale is set to 992mb (QFE, by definition).

Thereafter, the maths couldn't be simpler! You move the subscale from 992 to 1013 (i.e. by 21mb) and the altimeter display changes from zero to 630ft (i.e. by 21 x 30). The altimeter has now changed from showing height above aerodrome elevation (i.e. 0ft) to the aerodrome pressure altitude (i.e. 630ft). QED.

Don't be misled by the questioner, bose-x or friendly flier, your understanding is now fine. Good luck with the exams.

[Checkboard]

I might also point out that your diagram in post #30 has a few errors, Dave.

The line you have labelled "1013ft", I assume you mean 1013 hPa. As it is a higher pressure than the pressure you have placed (correctly) at the field, it should be lower in the diagram than the field, not higher. (In fact, it should be between the field and the sea.)

For altimetry, "wind on pressure = wind on height." To wind on 21 mb, we wind on 630 feet. (that's wind as in wound-up, not wind, as in blow )

[DaveD]

Ahh yes, i meant Hpa, and it was only a VERY rough paint job, worthy of the tape gallery if I do say so myself

[beatnik]

I think both Bose-x and Islander2 are both correct but are talking at cross purposes to each other.

Bose-x has worked out the Pressure Altitude (from Sea Level). Islander2 has worked out Aerodrome Pressure Altitude - which is what the question wanted, but is a term I've never heard of before (and probably why Bose-x's computer thingies haven't either). I'm not quite sure why you would want to know the Aerodrome Pressure Altitude other than to say to yourself that today my aircraft performance will be 630ft worse today than a "standard" 1013 day.

Surely the only relevant PA is that from Sea Level - so instead of the aerodrome being at 968ft on a "standard" day - today it now feels like it is 1,598 ft = therefore less performance, longer take off run etc.

Now I'll confuse the issue. I tried to work it out backwards. What is the QFE of this aerodrome on a standard day? Well 968ft / 30 = 32mb. So on a standard day of QNH 1013mb, QFE would be 1013-32=981mb. But today it is 992mb which is a HIGHER pressure reading, and therefore should provide BETTER performance - so why are Bose-x (1,598ft vs 968ft) and Islander2 (630ft vs 0ft) both showing worse performance. Where have I gone wrong?

[ft]

beatnik,
Islander is showing you the correct answer. The pressure altitude would be 630'. This is lower than the actual altitude, giving you slightly better performance due to the, as you correctly deduce, higher than standard pressure and thus lower density altitude.

The other calculation showed 1598', which is wrong and would in fact indicate lower performance in spite of the higher than standard pressure.

I don't know where you got the 630' vs 0' from, but that's the source of your confusion. In actual fact, it is 630' vs 968'.

Rgds,
/Fred

[Islander2]

Where have I gone wrong?

Sorry to say, in a whole lot of places!

Bose-x has worked out the Pressure Altitude (from Sea Level)

No he hasn't, he has worked out the pressure altitude assuming the sea level pressure is 992mb, which it isn't!

I'm not quite sure why you would want to know the Aerodrome Pressure Altitude other than to say to yourself that today my aircraft performance will be 630ft worse today than a "standard" 1013 day

No, on a 1013 standard day, the pressure altitude at the aerodrome elevation (the 'Aerodrome Pressure Altitude') would be the same as the physical elevation, i.e. 968ft. On the day in question, the sea level pressure at 1024mb is higher than standard and so the pressure altitude is lower at only 630ft and, leaving aside temperature effects (we really want to know density altitude, of course), the aircraft performance will be improved not worsened!

so instead of the aerodrome being at 968ft on a "standard" day - today it now feels like it is 1,598 ft = therefore less performance

No, as above it actually feels like 630ft = better performance.

so why are Bose-x (1,598ft vs 968ft) and Islander2 (630ft vs 0ft) both showing worse performance.

Only bose-x is showing worse performance and that's because his calculation wrongly uses 992mb as the QNH rather than the QFE. As above, my calculation shows the pressure altitude to be lower than the actual altitude, with a consequent improvement in performance (assuming it isn't markedly hotter than standard).

[beatnik]

I think I've got it now !

Thanks ft and Islander2.

[DaveD]

Thought i'd add, i checked my book tonight for a similar type question and to my delight i found one.

This confuser also gives you a reason to the answer....

It also added the difference between aerodrome altitude above amsl to the difference between the QFE and the standard 1013....


Now, im trying to understand why.

It's basically saying the pressure altitude is the aerodrome altitude above amsl plus the difference between QFE and 1013...

I cant get this wrong in the test, so I need to know for sure what is right.. What am I to believe... 2 sources which give the same answer, 1 of which gives a reason why, or PPRUNE?

[Checkboard]

Bose-x has worked out the Pressure Altitude (from Sea Level). Islander2 has worked out Aerodrome Pressure Altitude - which is what the question wanted, but is a term I've never heard of before (and probably why Bose-x's computer thingies haven't either). I'm not quite sure why you would want to know the Aerodrome Pressure Altitude...

Quite a few performance charts have pressure altitude and temperature as their entry arguments (in order to save you working out density altitude.). I am surprised you haven't heard of it!

The difference between the QFE (the Field Elevation pressure) and the standard 1013 hPa level (multiplied by the standard lower level correction of 30feet/hPa) is the Aerodrome's Pressure Altitude.

[ft]

Believe a standard atmosphere table.

The actual ratio of altitude change to pressure change is closer to 27 feet per millibar.

Using this, you get a pressure altitude of 27 * 21 feet, or 567 feet.

Compare this with the pressure at 570 feet in a standard atmosphere table: 992.6 mbar.

http://www.digitaldutch.com/atmoscalc/table.htm

It is amazing how poor the study material for would-be pilots often is.

Cheers,
/Fred

[Islander2]

The actual ratio of altitude change to pressure change is closer to 27 feet per millibar.

ft, I can see you have a good understanding of this topic, but might I add a note of clarification for those that don't and may take away the wrong message from your accurate observation as far as this question is concerned?

The ratio of altitude change to pressure change increases as you go higher.

Whilst 27ft/mb is indeed a good figure at low level, i.e. sea level to 1,000ft, 30ft/mb is a better average in the range 1,000 to 5,000ft ... and by the time you're worrying about the impact between 25-30k ft, the figure has grown to >65ft/mb!

[S-Works]

It's basically saying the pressure altitude is the aerodrome altitude above amsl plus the difference between QFE and 1013...

It's what I always understood hence my posting. I am still unclear as to why it is apparently wrong. But such is life.

[DaveD]

It's a bit of an ambiguous question.

On the one hand they give you the QFE, so you immediately think that your altimeter will be reading 0 on the altimeter, which is correct!

But what it's actually wanting for an answer is the aerodrome altitude plus the differences etc. Now which one is correct?

I would say both.. but obviously only one can be correct. So I'm going to go with the addition method at the end, which is also bose-x's method.

Islander isn't wrong in what he's saying, it's just a very poorly written question. It's worth noting that if the QFE is higher than the standard setting of 1013, then a subtraction will be used at the end.

[Islander2]

Dave

The question isn't in the slightest ambiguous.

What you've now concluded is that, on a high pressure day (QNH = 1024mb), an aeroplane at an aerodrome located 968ft above sea level will perform as if it were actually at 1,598ft above sea level.

There's nothing ambiguous about that, it's just plain wrong! Oh, and another thing, by concluding that the aeroplane is 1,598ft above the 1013mb pressure level, you're saying that it's 1,928ft above sea level (1,598 + 30x(1024-1013))!

Pressure altitude is what the altimeter will read when the subscale is set to 1013mb, nothing more, nothing less and no possibility for two sets of answers. You had already worked out yourself that there was no way the altimeter would read 1,598ft at aerodrome level on that day when set to 1013. You correctly concluded it would read 630ft. So what do you now think the 1,598ft means?

Believe it if you want to, but it is dangerously wrong thinking and I would strongly advise you to seek out some seriously better informed opinions before you ever try performance-limited take-offs from hot and high airfields.

It's worth noting that if the QFE is higher than the standard setting of 1013, then a subtraction will be used at the end.

You'd have to wait a long time at that airfield for those conditions ... it would need a sea level pressure >1045mb!!!!!!!

[S-Works]

where did performance calcs come into a discussion of pressure altitude?

[Checkboard]

Because it's the only reason you calculate pressure altitudes in the first place!

Flying with 1013 set (i.e. at Flight Levels) is a different subject, and is only done to simplify aircraft separation.

[Islander2]

where did performance calcs come into a discussion of pressure altitude?

I'm coming around to the view this is a wind up. But just on the off-chance you really didn't know, bose-x, density altitude is pressure altitude corrected for non-standard temperature. Oh, BTW, density altitude is what we use in performance calculations ... so performance calculations require the input of pressure altitude ... oh, WTF!

I really have reached the end of the road on this one. Just to summarise for Dave, though, he has apparently concluded that there are four potentially appropriate altimeter settings. Using the example of his original question (where we are told the airfield elevation is 968ft, the QFE is 992mb and the pressure lapse rate equates to 30ft/mb), when he sits in his aeroplane doing his pre-take-off checks he can:

a) set the altimeter to 992mb, the QFE, wherein it will show the height above airfield elevation and read 0ft;

b) set the altimeter to 1013mb, the standard altimeter setting, wherein it will show the pressure altitude (whilst on the ground, this is the aerodrome pressure altitude or QNE) and read 630ft;

c) set the altimeter to 1024mb, the QNH, where it will show the altitude above sea level and read 968ft; or

d) set the altimeter to 1045mb, the noise attenuating pressure setting, where it will show the altitude above an arbitrary and irrelevant reference and read 1,598ft (since this altimeter pressure setting is not defined in aviation, I've labelled it after a well-known headset!).

[DaveD]

I understand entirely what your saying Islander.

I'm not going to get a mark on an exam by saying the question is wrong though. So going with Bose-X's method will get me a mark, even though it's not right.. lol

[DaveD]

JUSTICE!

Sent an email to PPLQUIZ explaining my predicament with this question.

Dave

Thank you for your email.

You are correct, the question has now been changed to read:

An aerodrome elevation is 968ft and the QNH is 992mb, what is the
aerodrome pressure altitude? (1mb = 30ft).

With the answer being 1598ft

30 x 21 = 630ft

The QNH is lower than 1013 so you have to wind the Altimeter up to get
to 1013 so:

968 + 630 = 1598ft

Hope this make sense.

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