SlimHeader

×
PPL Video Lectures (10 Jul 2020)

PPL Video Lectures covering Aerodynamics, General Knowledge, Performance, Meteorology And Navigation are now available through our website see front page for details.

× Welcome to the CPL Performance question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.

Using density height for p charts

  • Posts: 122
  • Thank you received: 0

MissSoph created the topic: Using density height for p charts

Just going over my p charts…I only use the density height… and therefore the ISA temp for my take off chart only? If I was using the landing chart…would I also assume the same…. Use the density height if given, and the ISA temp? Or is that only a rule for take off?
#1

Please Log in or Create an account to join the conversation.

  • John.Heddles
  • John.Heddles's Avatar
  • Offline
  • ATPL/consulting aero engineer
  • Posts: 643
  • Thank you received: 69

John.Heddles replied the topic: Using density height for p charts

Not too sure exactly what your question intends. In particular, you refer to Or is that only a rule for take off? Whence comes that idea ?

Overall, though, one is best served by using the chart according to how the chart is constructed. One should be able to assume that the chart complies with the design standards for the relevant class of aircraft and country of origin.

So, if the chart provides for pressure height and OAT entries, use those (presuming you have them).

However, if you are, for instance, only given density height in the question, then you need to fiddle the entry somewhat to accommodate that.

On some charts, you will have an explicit density height scale - no problem - enter with the density height.

If you don't have a density height scale, about the only thing you can do is enter at the same pressure height as the given density height, along with the ISA temperature for that pressure height. Be aware, though, that in some charts, the Hp/OAT entry doesn't quite match up with Hd so there is a question mark in respect of compliance with the Australian rule if you just use Hd.

One other caveat. I suggested above that one should be able to make some assumptions regarding design standards. Unfortunately, CAO 20.7.4 was based on the Australian design standards in (the now defunct) CAO 101.22. Now that 101.22 is long gone, 20.7.4 is (incorrectly) used as a defacto design standard in Australian parlance. With respect to landing charts, for example, this can produce some problems.

20.7.4 imposes a requirement for landing -

5.1 Except in an emergency, an aeroplane must not land at a weight in excess of the least of the weights determined in accordance with subparagraphs (a) and (b):
(a) a weight at which the landing distance required in accordance with subsection 10 for the pressure height, temperature, runway slope (if in excess of 1%), and wind component along the runway at the time of landing, is equal to or less than the landing distance available in the direction of landing. Approved declared conditions may be used instead of actual pressure height and temperature;
(my bolding/highlighting)

However, with the changes, post Yates Report, Australia now adopts foreign documentation. Problem is that, sometimes, the foreign rules are different to the old Australian rules and the incoming foreign POH material doesn't comply with the 20.7.4 requirements. So, for instance, you might find some foreign landing charts having no temperature input. For instance, if I look at a FAR 23 requirement (which also has varied over the years so you need to tie a particular aircraft back to its frozen design standards), you might find

23.2130 Landing.
The applicant must determine the following, for standard temperatures at critical combinations of weight and altitude within the operational limits:
(again, my bolding/highlighting)

a requirement which presents a problem for Australian pilots.

How do you get around these concerns ? Not always easy to do. In this case, you can use density height, which will provide a more or less correct answer (per 20.7.4) for a light piston aircraft but whether that would satisfy the Judge in court after a mishap is another matter altogether.

A bit of background -

This was one of the benefits of the old DCA P-charts (eg as used in the Echo) which were part of the standard DCA Civil Mk 1 and Mk 2 flight manual formats used extensively prior to the Yates Report.

The P-chart format was developed and established by DCA (an antecedent of CASA) many years ago. I never bothered to find out, exactly, but I presume that the original legwork was done by John Fincher probably in the early- to mid-60s (the process was tied up to a very early HP computerised plotter). John is now long retired in Gippsland - I haven't spoken with him for some years so I can only hope that he is still with us and in reasonable health. Subsequently, he was DCA's performance engineering boss for many years - an extremely pleasant and competent engineer. The internal DCA reports which defined the processes were credited to two of his subordinates.

The advantage to the Industry was that the approach was a little bit conservative (but certainly not excessively so) and quite standardised so that a pilot could jump from one aircraft to another and know how to use the P-charts quite comfortably. For those of us who produced these charts, it was a step A, B, C process as one could just follow the reports and DCA would then accept the end charts without any fuss.

Engineering specialist in aircraft performance and weight control.
#2

Please Log in or Create an account to join the conversation.

  • John.Heddles
  • John.Heddles's Avatar
  • Offline
  • ATPL/consulting aero engineer
  • Posts: 643
  • Thank you received: 69

John.Heddles replied the topic: Using density height for p charts

Just as an aside - post Yates Report most of the old P-charts were consigned to the bin to be replaced by the original country of origin charts.

Although I haven't checked, the only likely aircraft in which you might still find the old P-charts, possibly, are the few which were originally certificated in Australia, for example, Victa Airtourer and TransAvia Airtruk

For the exam it really doesn't matter, the various format charts all do much the same thing - a bit like driving a Holden or a Ford, I guess.

Engineering specialist in aircraft performance and weight control.
#3

Please Log in or Create an account to join the conversation.

Time to create page: 0.160 seconds