Stuck!

[DaveD]

Stuck on a question on my flight planning... can someone explain how this graph works given the information etc?

Hope this is the right place to post this....

ImageShack - Image Hosting :: helpsu9.jpg (http://img213.imageshack.us/my.php?image=helpsu9.jpg)

[S-Works]

I think you are missing some of the input data for that question.

[RTN11]

Based on the information given, it doesn't seem possible.

Dividing the total moment by all up mass gives the arm as 42in.

This is not on the graph.

[BackPacker]

Yep. Seems like a duff question.

CofG is 42 inches aft of datum which is way off the chart and my experience tells me that with a graph like that, even with a very unusual loading you cannot even get the CofG that far forward - unless you fill the back with helium balloons or something.

(Oh, and for your conscience: yes, this is the right place to post questions like this. Even better now that I won a Sharp LCD TV...:})

[Pilot DAR]

Hi Dave,

The image does not seem to come across legibly, but in general:

The weight and balance "envelope" as depicted is showing you the maximum permissible center of gravity limits relative, and up to, the maximum gross weight. The actual aircraft weight for the proposed flight, once known by calculation, is plotted up the left side of the graph. Using the calculated C of G (from loading charts provided elsewhere for you, you will have calculated the total aircraft C of G position (doing so may require calculating the moment too, and working backward a bit). You then locate the C of G position along the weight line which you have identified on the chart.

If the plotted C of G position lies outside (left or right) the "envelope" entirely, flight is not permitted. (if it's too high, you're too heavy!) If the plotted point lies inside the "normal" envelope, you may fly the aircraft as a normal category aircraft, and only perform the manuevers stated in the limitations for that type of flight. If the plotted point lies within the "utility" portion of the envelope, additional manuevers (probably including spins) will be permitted. This is to be taken to mean that spins would not be permitted if the C of G point is in the normal envelope. Generally, if it is in the "utility" you may also fly as a "normal" category, though you might be having seat occupancy limitations.

Refer to the actual flight manual (if applicable) for the real information, as what I have told you is only the concept.

If that does not answer the question, let us know in greater detail what you need to know.

Pilot DAR

[wsmempson]

Something wrong with those figures - I suspect that the 58800 figure is wrong.

[DaveD]

This is exactly what i thought...... total mass of the plane divided by the total moment should give you the moment of the datum, but 42 shows nowhere on the graph... that's why I was confused... thought I was doing something wrong, couldn't even find it in my PPL manuals.. :ugh:

[S-Works]

As I said before, I suspect that you may be missing more elements of the question. Or as has been mentioned the moment data could be wrong.

[DaveD]

That's off a website designed for your ppl. That's also everything that came with the question..

[RTN11]

Is there any sort of forum associated with that website, or contact details for the people who run it?

I would flag this up as a dodgy question, and see what they say.

[DaveD]

I could try, other than that question the site is very good.

www.pplquiz.co.uk

[Lister Noble]

Dave,I tried the demo and it seems a good site although I found an error with another unrelated question but can't remember what it was now
I used www.airquiz when I was a student and found it to be very useful.
Good luck
Lister

[RTN11]

Do you have a PPL confuser?

I found mine very useful when I did my PPL.

[DaveD]

Yeah, got one of those too.

[Cusco]

It's a dud question: Bin it and move on............

Cusco.

[DaveD]

ImageShack - Image Hosting :: helpja1.jpg (http://img4.imageshack.us/my.php?image=helpja1.jpg)

Can anyone tell me the calculations to working this question out?

[S-Works]

1013-992 = 21mb

21mb x 30 = 630ft

630 + 968 = 1598ft

[DaveD]

Edit... PPLQuiz giving me wrong answers again. Im presuming the last caluclation of an addition is correct?

PPLQUIZ said 338 was the right answer :bored::bored::bored:

[S-Works]

What is pressure altitude Dave?

Pressure altitude is the height in the standard atmosphere above the 1013.2mb (hpa) pressure level at which the pressure equals that of the aircraft or point under consideration.

If you were on the ground you would find your pressure altitude by setting 1013 on the altimeter subscale. So you need to add any difference between your current setting and 1013 if lower or deduct it if higher.

[Islander2]

Well, bose-x's calculation is wrong ... as are all of the possible answers given!

Pressure altitude is what the altimeter reads if 1013mb is set on the sub-scale.

If the altimeter reads zero when 992mb is set, it will read 630ft with 1013mb set.

The airfield elevation is a distractor in this question. Unfortunately, it also distracted the questioner!

[S-Works]

Uh? Where does it say the altimeter is set to zero?

I just ran the question through my E6B and it came up with the answer that I gave.

The question does not ask what the altimeter would display it asks the pressure altitude. In which case I am pretty sure that both the question and my answer add up....

[Islander2]

Where does it say the altimeter is set to zero?
What does QFE mean?

[S-Works]

I think you need to read the question.

[Islander2]

bose-x, I did!

The question as stated is 'what is the pressure altitude at an aerodrome where the QFE is 992mb.' The only possible answer (at 30ft per mb) is 630ft.

[S-Works]

I just put the question through my E6B on my iPhone and the ASA CX2 pathfinder and the answer came up as 1598ft.

So one of us is not reading the question correctly.....

The question:

An aerodrome elevation is 968ft and the QFE is 992mb, what is the aerodrome pressure altitude?

A 1598ft
B 338ft
C 720ft
D 1270ft

[Islander2]

Or one of us doesn't really understand altimetry! ;)

[S-Works]

Or one of us doesn't really understand altimetry!

Clearly, which also apparently means none of the software developers who have written the software I have plugged it into have an understanding either.....

[Islander2]

Let's try phrasing the question slightly differently.

If the outside air pressure is 992mb, how high are we above 1013mb?

[DaveD]

Your right bose-x, its much easier if you draw it out as a diagram..

[DaveD]

http://img255.imageshack.us/img255/9189/diagramap1.th.jpg (http://img255.imageshack.us/my.php?image=diagramap1.jpg)


This looks right to me.. maybe an easier way to view it?

[S-Works]

Yep, looks right to me. Perhaps my old fashioned view of Altimetry as opposed to Islander2's works?

[Islander2]

bose-x, I'll give you a clue.

Your E6B and CX2 are suffering from garbage in == garbage out!

You are misreading aerodrome QFE as aerodrome QNH.

[S-Works]

As you wish mate. My E6B has to 2 entries for a pressure altitude question.

Pressure - Pressure is 992
Altitude - 968ft

My simple understanding is that if you are 968 ft and the pressure at 968 ft is 992 if you add 21mb to that then the pressure altitude is 1598 ft. You have added 630 feet by taking it to 1013 which gives you the pressure altitude. Pressure altitude being a common datum rather than QNH.

The question does not say anything about an altimeter it just asks what the pressure altitude is based on the figures given.

I just plugged in the 2 figures from the question and out popped an answer of 1598ft which was the same as my original old fashioned calculation and as answer A on the quiz.

I clearly do not understand how you are working it out. But please feel free to edify us.

[Islander2]

I clearly do not understand how you are working it out. But please feel free to edify us.I thought I had!

All you need to do is answer my rephrasing of the question:
If the outside air pressure is 992mb, how high are we above 1013mb?... and then explain why you think that's a different question from the one posed.

I say again, your answer is based on the the 992mb pressure being at sea level ... whereas the question says that it is the pressure at aerodrome level.

[S-Works]

You have lost me completely.....

[Islander2]

Ah well, I'll bail out then. It only remains for me to wish Dave luck in his exams ... you can still pass even if you get the altimetry questions wrong!

[DaveD]

I understand it like this.


Your stood at an aerodrome which is 968ft above mean sea level. You have an altimeter in your hand. In order for it to read 0 then you must set 992mb.
Now, the question is asking for the aerodrome pressure altitude.

"pressure altitude is the indicated altitude when the altimeter is set to an agreed baseline pressure setting" In our case, this is 1013mb.

So, were stood at the aerodrome and it wants the aerodrome pressure altitude. So to set 1013 from 992 we need to go up 21mb. 21mb at 30ft per second is 630ft.

Now are we to believe because we have been given the QFE the altitude is 0, well yes.. Hmm interesting, whilst typing this out i actually understand what Islander is talking about...

[DaveD]

How the hell does PPL quiz get 338mb? That wouldn't be pressure altitude, that would be mean sea level altitude right?

[DaveD]

Explain why?

It wants the pressure altitude at the aerodrome.

So it wants the altitude when you set the altimeter from QFE of 992mb to 1013mb..

[Islander2]

Dave, I'm heartened, you've got it!

Their answer of 338ft is a nonsense. That is actually the height of the 1013mb pressure level above MSL (QNH is 1024.27mb), which is a meaningless number.

[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 thoughDon'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 :D

[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 dayNo, 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 performanceNo, 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. :ok:

[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. :confused:

[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))!:D

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!!!!!!!:D

[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.

Most of our questions come from student feedback, which you can enter
yourself by going to the Feedback page.

If in the future you do come across a question which you feel needed
challenging please use the ID box on the top right hand side so we can
locate the question easier.

Kind regards

PPLQuiz

[S-Works]

I guess I just read correctly into the question rather than interpreting it to literally. I guess sometimes it pays to have an open mind.

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


Indeed but the question was not around performance it was a pretty simple question that it seems had a typo in it. Thanks for the condescending lecture though.

I just applied logic to see around the typo it appears some were unable to.

[Checkboard]

I just applied logic to see around the typo it appears some were unable to.

:rolleyes:

In instructor parlance we call that statement "a justification" :}

[Islander2]

I just applied logic to see around the typo it appears some were unable to.Just out of interest, how come the logic you applied to 'see around the typo' got you a different answer to their's (which was 338ft)?:D

Only asking, as they say. ;)

As you observed, it was a pretty simple question ... so the inability of an instructor to distinguish between QFE and QNH seems rather fundamental, especially after the error he was making had been pointed out on several occasions.

[S-Works]

I got the same answer as them and gave the correct math for the problem. Perhaps I read the question in the right context rather than looking at it dogmatically. So I would probably say it is the ability of an Instructor to look at the big picture...

However as my teenagers say - 'whatever'......

1013-992 = 21mb

21mb x 30 = 630ft

630 + 968 = 1598ft

[Islander2]

I got the same answer as them Ermm, no. As DaveD said in response to your answer:

PPLQUIZ said 338 was the right answer http://static.pprune.org/images/smilies/wbored.gifhttp://static.pprune.org/images/smilies/wbored.gifhttp://static.pprune.org/images/smilies/wbored.gif

[S-Works]

Not what he says below:

But like I said whatever, we will have to agree to disagree.


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.

Most of our questions come from student feedback, which you can enter
yourself by going to the Feedback page.

If in the future you do come across a question which you feel needed
challenging please use the ID box on the top right hand side so we can
locate the question easier.

Kind regards

PPLQuiz

[ft]

ATC: ”B-OX, descend and maintain 3000 feet, QFE 995”

B-OX: ”3000 ft, QFE 995, B-OX”

(Thinking: Hey, QFE? They always give QNH around here… and I have an open mind, so I’ll just correct that without pointing out their obvious mistake, without declaring my own assumption and without interpreting things too literally. Lesse… they want me 3000 feet above AD elevation, and the AD is at 1400 feet. Allright, here we go, level off at 4400 feet. Man I’m on top of things!)

AAIB: ”A Piper PA28 and a B737 collided after a private pilot with an open mind failed to comply with ATC instructions in a non-radar environment. Five crew members and 78 passengers onboard the B737 were killed, along with the open-minded private pilot.”

Literally is how we do things in this industry. It is a method of reaching old age which has been proven time and time again. If something does not make sense taken literally, we question it but do not, repeat, do not go off acting upon our own assumptions.

And if we mess up, we call the tower upon landing on the telephone number they kindly provided, apologize and admit our mistake. After all, we are all human and we all make mistakes. Trying to deny having made a mistake is generally not a good idea, especially if it is on record.

[S-Works]

Oh god.....
:ugh::ugh::ugh::ugh::ugh: