MORE PLANE ENGLISH ON PART 91

Being an engineer I like to take an interest in numbers so let’s discuss one of the “matters to be checked before take-off”: take-off and landing performance. Let’s limit the scope of the discussion to small single-engine aeroplanes certified to FAR 23 and that is important for several reasons:

  • There is nil requirement for any performance limitations and any operating limitations must be specified in the airplane flight manual (look in the limitations section).
  • FAR 23 specifies the minimum performance information which must be provided to the pilot and that depended on which version of FAR 23 the airplane was certified to. Manufacturers don’t continually revise their manuals for airplanes that were built many years ago, especially if it was a different company that built them under the same type certificate.
  • There are different amendments of FAR 23, for example, earlier versions required only that take-off and landing may not require exceptional pilot skill and later ones “not require more than average pilot skill”. There is a difference in the way a test pilot addresses each!

The Sept-Oct 2002 issue of Flight Safety Magazine included an article which addressed these issues:

“….. It is tempting to simply say the whole episode could have been avoided if the pilot had consulted his aircraft’s take-off performance charts … However, it is unrealistic to assume that all light-aircraft pilots will calculate the exact take-off and landing distance required before every flight. ….… A word of warning about aircraft performance charts. ….. the production of uniquely Australian charts ceased and pilots …… calculate performance using information supplied by manufacturers. 

Aircraft manufacturers’ performance charts do not include built-in safety factors and in most cases reflect best-possible performance achieved with: Highly experienced test pilots …”. All true!

THE WAY WE DO IT NOW

CASA’s VFRG gives the current rules for take-off and landing distance determination in checking the maximum take-off weight.

“it is acceptable to base all take-off and landing weight limitation calculations on declared meteorological conditions …. alone and you may only be required to determine weight limitations three times per year (for summer, winter and autumn/spring seasons).” i.e. the current rules give that choice of simply using the declared density height at your location.

Let’s use a new Piper Archer III as an example. The POH states: “The performance charts are unfactored and do not make any allowance for varying degrees of pilot proficiency or mechanical deterioration of the aircraft.” It was certified to CAR 3 (which preceded FAR 23) plus a small selection of requirements from earlier versions of FAR 23. It seems to me that the POH is trying to say that it will not be easy for the average pilot to achieve the take-off and landing distances quoted.

The Piper Archer III POH goes on to state: “This performance, however, can be duplicated by following the stated procedures in a properly maintained airplane.” So it is possible to achieve those distances if you nail those airspeeds and your ASI has nil error. Of course, the airframe must be in reasonable condition and the engine must be developing rated power. Check the slow idle RPM as anything higher than Lycoming’s recommendation will lengthen the landing distance.

The usual CASA tolerance of up to 5 kts over the specified speeds will result in a significant increase in distance and there is no margin in the POH chart to cater for it. The FAA’s excellent Handbook of Aeronautical Knowledge Chapter 11, Aircraft Performance states that “ten percent excess airspeed would increase the takeoff distance 21 percent”! It is worthwhile reading that chapter to fully understand performance data in the POH for an American aircraft.

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/13_phak_ch11.pdf

The first thing that many pilots would notice about this is that it is different than what they used in their theory course. As noted in that 2002 Flight Safety Magazine article those uniquely Australian “P Charts” were withdrawn many years ago.

The Archer 3 POH usefully states: “Effects of conditions not considered on the charts must be evaluated by the pilot, such as the effect of soft or grass runway surface on takeoff and landing performance …”.

Those “P Charts” included the effect of different runway surfaces and runway slope but the Piper chart above does not have that information.

The current CAO 20.7.4 allows us to comply with the requirements of the POH so how does a pilot make that evaluation? I like to use the UK CAA’s Safety Sense Leaflet 07 on Aeroplane Performance which you may find online.

https://publicapps.caa.co.uk/docs/33/20130121SSL07.pdf

I recommend that you read it now then return to my discussion as it has much good information and guidance.

The UK CAA is familiar with the issues regarding American performance data which I have outlined so this Leaflet recommends appropriate margins for you to use. The good news is that you don’t need to do work your way through the calculations in that Leaflet as it is incorporated in the GASCo Performance Calculator for your iPad or GASCo Performance mini for your iPhone.

If you are operating an aircraft with performance information similar to that of the Archer at a grass runway you could perhaps use that app as it will give data using a similar method to the old Australian “P Charts” plus a good safety margin. Disclaimer – read my later comments about this app below!

It is important to note that the Archer III POH does not allow extrapolation beyond the limits of the charts so you must not take-off above an ambient temperature of 50°C for example.

The manual for my Super Decathlon instead has guidance such as “Good pilot judgement must be used under all conditions …”. The absence of data for older types is not necessarily a limit on the operation.

THE GAME CHANGES WITH PART 91

CASA’s new plain English guide is supposed to be easily understandable however you really need to stop to think about it all as there are some significant differences from the current rules.

“Before take-off, you must complete the following checks.” I think that is very clear – we must do the checks stated next.

“… the aircraft’s take-off, en-route and landing performance capabilities meet the performance requirements required by, or under, the regulations for the circumstances and conditions expected during the flight.”

OK, so we must check take-off and landing distances for the specific situation on the day. Therefore, we may not use the declared density altitudes described in the current VFRG.

It goes on: “You must determine the performance capability of the aeroplane or rotorcraft at the take-off weight, and you must not exceed the weight limitation contained in or derived from either:

          the AFM

          the manufacturer’s data manual (if any), or

          other data approved for the purpose”

As you learnt in your pilot theory course, you must determine that the take-off and landing distances required are no greater than the distances available.

The Archer III has that information in the AFM (which is incorporated in the POH). My Super Decathlon has some information in the manufacturer’s manual so neither checked nor approved by the airworthiness authority and therefore warrants every bit of the safety margin recommended by Leaflet 07.

WHAT IS DATA APPROVED FOR THE PURPOSE?

The big new issue that I see is if you are operating from a grass runway or a wet runway or a runway with some slope then you probably don’t have that data in the POH. The data for my Super Decathlon and some other types also do not have any information on the effect of a slight tailwind.

Those old uniquely Australian “P Charts” did include the effect of a tailwind as well as runway slope and different runway surfaces. CASA withdrew their approval for them many years ago. They are old so are simply not valid for many aeroplanes built since then.

The UK CAA’s Leaflet 07 is not approved by CASA that I am aware of.

The iPhone app is not approved as it clearly states that it has “been drawn from” the Leaflet but “a number of assumptions have been made” and “this product is not certified as, nor intended as, a flight planning tool.”

If the appropriate data is not supplied by the manufacturer then, per the new Part 91, we must have “other data approved for the purpose”.

IS THIS A PLAIN ENGLISH GUIDE

I accept that it is in plain English and it is a guide to what is in the regulations: “By following this guide, it is expected you will comply with the aviation regulations“. However, the regulations are much more prescriptive and onerous than the equivalent regulations in the USA so do not fit well with aircraft manuals developed for the American regulations as they were many years ago.

It is plain to me that after March 2021 I must change my flying operations as I do not have “other data approved for the purpose” of:

  • Determining take-off and landing distance on a grass airstrip.
  • Determining take-off and landing distance with a slight downwind component. When the Tower advises me there is a very small downwind component or that the runway is wet I must decline the takeoff clearance. If inbound then I would be obliged to declare an emergency.

Perhaps my opinion is incorrect so I look forward to an Advisory Circular on this subject. “Advisory Circulars are intended to provide advice and guidance to illustrate a means, but not necessarily the only means, of complying with the Regulations, or to explain certain regulatory requirements by providing informative, interpretative and explanatory material.” That should sort us out as I do not see any means for many of us to comply with Part 91.

Perhaps the intended means of compliance will be to bring back the unique Australian “P” Charts and get them approved at an enormous cost?