Go Back  PPRuNe Forums > PPRuNe Worldwide > The Pacific: General Aviation & Questions
Reload this Page >

Density height calculation question

The Pacific: General Aviation & Questions The place for students, instructors and charter guys in Oz, NZ and the rest of Oceania.

Density height calculation question

Old 7th Nov 2020, 04:58
  #1 (permalink)  
Thread Starter
 
Join Date: Nov 2020
Location: Aus
Posts: 3
Density height calculation question

I have an assessment for the Qantas pilot academy. They have sent me a practice test with the following question:

"The elevation of Echo is 3480ft AMSL. The QNH is 1021 hPa and the temperature is 24C. What is the density height of Echo?"
A. 1920 ft
B. 5105 ft
C. 3820 ft
D. 5560 ft

I calculate this as follows:

PH = 3480 + (1013-1021) x 30))
= 3480 - 240 = 3240 ft PH

ISA Temp @ 3240 PH = 15 + (3 x -2) = 9C

ISA Deviation = OAT - ISA
= 24 - 9 = +15C

Density Height = PH + (ISA Deviation x 120)
= 3240 + (15 x 120)
= 5040 ft DH

Can someone please explain what I'm doing wrong?
MB00086 is offline  
Old 7th Nov 2020, 05:43
  #2 (permalink)  
 
Join Date: Sep 2002
Location: Great South East, tired and retired
Posts: 3,483
Can someone please explain what I'm doing wrong?
Applying for the Qantas Academy is what.
Ascend Charlie is online now  
Old 7th Nov 2020, 05:49
  #3 (permalink)  
 
Join Date: Mar 2017
Location: Cairns
Posts: 19
Originally Posted by MB00086 View Post
I have an assessment for the Qantas pilot academy. They have sent me a practice test with the following question:

"The elevation of Echo is 3480ft AMSL. The QNH is 1021 hPa and the temperature is 24C. What is the density height of Echo?"
A. 1920 ft
B. 5105 ft
C. 3820 ft
D. 5560 ft

I calculate this as follows:

PH = 3480 + (1013-1021) x 30))
= 3480 - 240 = 3240 ft PH

ISA Temp @ 3240 PH = 15 + (3 x -2) = 9C

ISA Deviation = OAT - ISA
= 24 - 9 = +15C

Density Height = PH + (ISA Deviation x 120)
= 3240 + (15 x 120)
= 5040 ft DH

Can someone please explain what I'm doing wrong?
Using more accurate 3.24 instead of 3 in the ISA dev formula gets a final answer of 5098ft which is closer to (B)
havoste is offline  
Old 7th Nov 2020, 06:10
  #4 (permalink)  
Thread Starter
 
Join Date: Nov 2020
Location: Aus
Posts: 3
Yea that's what I did, and apparently B is the correct answer. It's just in all the study material it tells me to round to the nearest 500ft to calculate temp drop. I was expecting as it's multiple choice I would get the exact answer.
MB00086 is offline  
Old 7th Nov 2020, 06:52
  #5 (permalink)  
 
Join Date: Dec 1998
Location: Escapee from Ultima Thule
Posts: 4,271
ISA temp is 2*C / 1000' which is 0.5*C per 250'.

So ISA T* at 3240' is nearer 8.5*C, not 9*C

That gives 5100' for an answer, so B
==========
Or using my CR whiz wheel a fraction over 5000' which is also more than close enough to pick B
Tinstaafl is offline  
Old 7th Nov 2020, 07:46
  #6 (permalink)  
 
Join Date: Jul 2005
Location: ipswich
Posts: 54
Wait till you do some plotting, the thickness of the pencil they used for the questions determines the answer. :-(
binzer is offline  
Old 7th Nov 2020, 07:58
  #7 (permalink)  
 
Join Date: May 2005
Location: France / Qatar
Age: 66
Posts: 1,035
PH = 3480 – [(1021-1013)x30]

= 3480-240

= 3240

ISA at 3240 = 15-(2x3.2)

= 15-6.4

= 8.6

ISA dev = 24-8.6

= 15.4

Density Alt increases approx. 1000ft for every 8C above ISA

Therefore Density Height of Echo = 3240+[(15.4/8)x1000]

= 3240+1925

= 5165, which is pretty close to ‘B’.
eckhard is offline  
Old 7th Nov 2020, 08:15
  #8 (permalink)  
 
Join Date: Aug 1999
Location: Gold Coast, Australia
Age: 73
Posts: 4,305
Originally Posted by Tinstaafl View Post
ISA temp is 2*C / 1000' which is 0.5*C per 250'.

So ISA T* at 3240' is nearer 8.5*C, not 9*C

That gives 5100' for an answer, so B
==========
Or using my CR whiz wheel a fraction over 5000' which is also more than close enough to pick B
One of the few bizarre things that actually took hold during my Met lessons sometime last century was the Dry Adiabatic Lapse Rate is 1.98C per 1,000ft. So the ISA Temp at 3,240ft is 8.5848*C rounded to 8.6*C, not 8.5*C. Pretty much what eckhard calculated.

That would round out the result a foot further from 'B'
John Eacott is offline  
Old 7th Nov 2020, 08:17
  #9 (permalink)  
 
Join Date: Nov 2001
Location: Australia/India
Posts: 3,715
The correct answer: "My calculations lead me to conclude that the answer is 5040'. I therefore don't understand where I may have gone wrong."

5,040' feet is, in principle, a 'less safe' answer than the 5,105' in B, as you're erring 65' on the 'dangerous' side for your planning. But, as binzer has observed, and as you will find in the real world, there are hundreds of feet in a pencil width and rounding assumptions. Just avoid any operation in which 65' either way make a real difference.

Qantas put a Jumbo into a golf course - or was it a rice paddy? I forget - but I reckon that 65' either way wasn't the cause.
Lead Balloon is offline  
Old 7th Nov 2020, 09:25
  #10 (permalink)  
 
Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 722
I don't think it is a case of what you are doing wrong. As they say in FAA land: B 'is the most correct answer'. I am not even joking.
oggers is offline  
Old 7th Nov 2020, 10:57
  #11 (permalink)  
 
Join Date: Nov 2008
Location: Fresno
Age: 71
Posts: 260
I've only ever used Density Altitude.
Thud105 is offline  
Old 7th Nov 2020, 12:15
  #12 (permalink)  
 
Join Date: Jun 2000
Location: Australia
Posts: 4,164
Looks like the type of questions haven't changed much since the old DCA SCPL theory exams of the 1950 - 1960's.

I well remember scratching my head around the complexity of gyro steering questions centred on the South Pole. After joining DCA in 1969 my office was next to Ted Steele the former wartime RAAF navigator who set the SCPL navigation and flight planning exams in those days. He came across as an embittered old man who hated pilots for some reason. I tackled him on the uselessness of South Pole gyro steering questions for the average GA charter pilot flying Chieftains to Tasmania which was SFA. He said the SCPL exams were first vetted and approved by Qantas as that was the standard demanded of Qantas recruits. That could be an urban myth but it came from the horse's mouth so to speak.
Centaurus is offline  
Old 7th Nov 2020, 12:50
  #13 (permalink)  
 
Join Date: Dec 2001
Location: FL, USA
Posts: 2,738
Closest answer, donít worry if itís not exactly one of the given answers.
You donít know if theyíve used 2C or 1.65 or 1.98 and 30í or 25í and if theyíve rounded up or down.
These are fun to do on the back of a paper napkin but in real life you use tables to reduce the possibility of errors.
That is if you fly analog piston aircraft.
Anything bigger and itís provided for you.
Pretty much.
B2N2 is offline  
Old 7th Nov 2020, 14:54
  #14 (permalink)  
swh

Eidolon
 
Join Date: May 2001
Location: Some hole
Posts: 2,062
Originally Posted by MB00086 View Post
I have an assessment for the Qantas pilot academy. They have sent me a practice test with the following question:

"The elevation of Echo is 3480ft AMSL. The QNH is 1021 hPa and the temperature is 24C. What is the density height of Echo?"
A. 1920 ft
B. 5105 ft
C. 3820 ft
D. 5560 ft

I calculate this as follows:

PH = 3480 + (1013-1021) x 30))
= 3480 - 240 = 3240 ft PH

ISA Temp @ 3240 PH = 15 + (3 x -2) = 9C

ISA Deviation = OAT - ISA
= 24 - 9 = +15C

Density Height = PH + (ISA Deviation x 120)
= 3240 + (15 x 120)
= 5040 ft DH

Can someone please explain what I'm doing wrong?
Essentially you have done nothing wrong, CASA does not require you to know that relative humidity/dew point also impacts density height. Their answer assumes a relative humanity of 85% or a dew point of -5 deg C.
swh is offline  
Old 7th Nov 2020, 15:02
  #15 (permalink)  
 
Join Date: Jun 2013
Location: somewhere
Posts: 59
Calc

For some reason, my computer asks to input dew point temperature, too. I left it that in blank and it came up with density alt 5043 ft, press alt 3268 ft, ISA 8.5C, and ISA Dev 15.5 C
🤔
Neektu is offline  
Old 7th Nov 2020, 16:23
  #16 (permalink)  
 
Join Date: Aug 1998
Location: Ex-pat Aussie in the UK
Posts: 5,331
So some bloke at Qantas has no knowlege of how to calculate density height, and simply plugged the values into a computer, without understanding the algorithm, and wrote down the resultant answer ...

Sounds about right for an airline training department.
Checkboard is offline  
Old 7th Nov 2020, 20:09
  #17 (permalink)  
 
Join Date: Sep 2003
Location: Somewhere on the Australian Coast
Posts: 1,013
Originally Posted by Checkboard View Post
Sounds about right for an airline training department.
What a load of codswallop. QANTAS has a pretty good training department from my experience, full of blokes (and even sheilas), with plenty of knowledge and both the desire and the ability to impart it to their trainees.

Itís a pretty fair bet they had nothing to do with the writing of this question either.
DirectAnywhere is offline  
Old 7th Nov 2020, 21:22
  #18 (permalink)  
 
Join Date: Aug 1998
Location: Ex-pat Aussie in the UK
Posts: 5,331
Not codswallop in the slightest. Ask an airline pilot to calculate an off-track PNR in flight sometime. Airline training does not review many ATPL subjects. Calculating a density height would be a head scratcher for most airline pilots - including trainers.
Checkboard is offline  
Old 7th Nov 2020, 21:30
  #19 (permalink)  
 
Join Date: Jun 2001
Location: FNQ ... It's Permanent!
Posts: 3,756
An airline pilot would have no need to work out density altitude. Just like they don’t need to use a sextant or do DR plots!

Capt Fathom is offline  
Old 7th Nov 2020, 23:43
  #20 (permalink)  
 
Join Date: Sep 2003
Location: Somewhere on the Australian Coast
Posts: 1,013
Originally Posted by Checkboard View Post
Not codswallop in the slightest. Ask an airline pilot to calculate an off-track PNR in flight sometime. Airline training does not review many ATPL subjects. Calculating a density height would be a head scratcher for most airline pilots - including trainers.
Youíre probably right - but that has stuff all to do with airline ops and airline training and nothing whatsoever to do with the level of involvement that QANTAS training would have had in this question - precisely zero. Your post was casting aspersions against QF training. Itís irrelevant.
DirectAnywhere is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright © 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.