Back in October 2019 I wrote about some perceived issues with the performance rules in the new Part 91.
See my AOPA Australia article here and the relevant posts on this website.
The final rule differed from the draft however the issues remained. As before let’s limit the scope of the discussion to small single-engine aeroplanes certified to FAR 23.
In that article I finished with the comment “Perhaps my opinion is incorrect so I look forward to an Advisory Circular on this subject.”
Now we have it! In October 2021 CASA published two relevant documents:
- ACCEPTABLE MEANS OF COMPLIANCE AND GUIDANCE MATERIAL General operating and flight rules Part 91 of CASR
- ADVISORY CIRCULAR AC 91-02 v1.0 Guidelines for aeroplanes with MTOW not exceeding 5 700 kg – suitable places to take off and land
I am going to discuss the situation in two parts. The first being the rules and the second being practical flight operations. The good news is that CASA has relieved my concerns with the new rules by publishing these documents.
PART 1: Take-off and Landing Flight Performance Rules
I recall the main points of my 2019 article:
- Any operating limitations are explicitly stated in the AFM.
- The scope of the flight performance information provided will vary depending on the particular amendment of FAR 23 that applied to the type. Only newly certified aircraft will have performance data for all conditions within the operational limitations of the aircraft. For aircraft certified earlier the range of conditions provided for flight performance does not limit the operations.
- The requirement to only use data from the AFM, the manufacturer’s data manual (if any), or other data approved for the purpose. This is particularly onerous so we’ll start with this and see how the two CASA references above deal with it.
I had suggested a couple of questions to ask. The first was:
Will CASA be providing guidance in the form of an AC to include aircraft with an older certification basis, or no certification basis, where the performance information provided is quite sparse? Only aircraft certified to later amendments of FAR 23 were required to provide performance data for all conditions within the operational limitations of the aircraft.
Reference 1 handballs 91.795 Take-off performance and 91.800 Landing performance directly to reference 2 however that reference is aimed at 91.410 Use of aerodromes. There is only this passing reference to:
“Regulations 91.795 and 91.800 stipulate that an aircraft must not take off or land above the maximum all up weight of the aircraft from the AFM (or equivalent), or a more limiting weight due to the aircraft performance requirements specified in the Part 91 Manual of Standards.”
The sections of those regulations which considered particularly onerous are not mentioned at all by reference 2 and it goes along with my own view of how the FAR 23 certification rules for flight performance should be applied to operations. Here are the relevant extracts:
“For aeroplanes with MTOW not exceeding 5700 kg where the information available to the pilot may be non-specific or incomplete, the use of suitable safety factors to mitigate these risks will maximise the safety outcome. …. The different certification standards specify what information must be provided in the AFM/pilot operating handbook (POH).”
That is as far as it goes on the subject of the 61.795 and 91.800 rules so it is quite sensible and consistent with my views. Reference 2 has much good guidance on how a pilot is to take what performance information is made available in the aircraft manuals and “If the AFM has no such guidance, it is recommended that pilots apply the allowances relevant to the circumstance described and shown in Tables 2 and 3.”
I had earlier stated that I like to use the UK CAA’s Safety Sense Leaflet 07 on Aeroplane Performance and the AC is very similar. Tables 2 and 3 of the AC provide information on the effect of pressure altitude, ambient temperature, tailwind, runway slope and runway surface.
PART 2: Take-off and Landing Flight Performance Application
Recalling the main points of my 2019 article:
- 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!
My second question was: The Part 91 MOS 24.02 states “You must determine the aeroplane performance from 1 of the following” then lists the AFM, manufacturer’s data manual or data approved under Part 21. Does this mean that only data from any of those three sources may be used or may other data be used to supplement what is in there? For example, the Piper Archer III POH 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 …”.
Reference 2 emphasises that “Regulation 91.095 requires the pilot to operate in accordance with the ‘aircraft flight manual instructions’.” So, stuff must be evaluated by the pilot and this seems to trump the rule in the Part 91 MOS that I was concerned about.
Reference 2 answers my question very well and has excellent guidance on the subject. Rather than discuss the content I will just consider two different examples of aircraft that I had mentioned earlier.
The Piper Archer III POH 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 …”. It is also 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.
So, if we want to take an Archer III into a grass strip with a slope then we may do it by applying the allowances in Tables 2 and 3 of the AC. All straightforward.
My second example is the 8KCAB Decathlon. The latest variant of the 8KCAB is the Xtreme Decathlon which was certified in December, 2012 and, unlike earlier variants, the AFM incorporates all of the performance data. The approved maximum outside air temperature limit is 120 deg F (49 deg C). The airfield performance data includes the effect of tailwinds and different runway surfaces.
Take-off and landing performance is only provided up to a maximum temperature of 104 deg F (40 deg C) as specifically required by FAR 23. 40 deg C is simply as far as the provided data goes. Pilots may choose to go flying when the temperature is up to 49 deg C .
The Super Decathlon variant of the 8KCAB was certified to an early version of FAR 23 and is still in current production. It retains the same take-off and landing distance data as the original from the 1960s. The absence of data for older types is not necessarily a limit on the operation due to the certification basis of the type. Certification flight testing is expensive and there is no requirement for the manufacturer to provide additional information for operators. The manual notes that
“This data is to inform the pilot what he can expect from the aircraft in the way of performance and to assist in flight planning.” It goes on. “Good pilot judgement must be used under all conditions …”.
Let’s consider a couple of typical operating situations:
An example similar to the Piper Archer III example above with an ambient temperature of 49 deg C going into a grass strip with a slope. The Super Decathlon has no relevant limitations in the AFM which would be a show-stopper. As with my first example we may do it by applying the allowances in Tables 2 and 3 of the AC. The tables include the effect of temperatures too. All straightforward again.
How about a typical day at my local towered airport where there is a slight tailwind? The manual simply states that it provides nil information about the effect on distance however the AC does now provide such data. A typical new POH may only provide for a small tailwind of up to 5 kts. The old Australian CAA “P” Charts, including that for the Decathlon, provided the same maximum tailwind. All straightforward again.
Again, tables 2 and 3 of the AC provide for the effect of a tailwind
The AC states: “The performance of every certificated aircraft has been evaluated as part of the certification process.” That is not true of types certified some years ago such as the Super Decathlon!
It also states: “… for the certification of landing distance, the requirements for the test are: …. the aeroplane must approach at not less than 1.3 times the stall speed in the landing configuration.” That is good but only true for later versions of FAR 23. The landing approach speed specified for the Super Decathlon is 52 kts which is very close to the stall speed of 49 kts.
Incidentally, the AFM for the Xtreme Decathlon has sensible airspeeds for take-off and landing as required by the later FAR 23.
The AC notes “Importantly, it is unlikely that a normal pilot can replicate the testing performance during routine flying conditions. The likelihood decreases even further when flying conditions become more challenging.” That is especially true of the Super Decathlon as FAR 23 at the time only required that it “must be able to be landed safely and come to a stop without exceptional piloting skill”. Later versions of FAR 23 changed to the more reasonable “The landing may not require more than average piloting skill”.
One of the most important aspects of replicating that test performance is to achieve the specified airspeeds. The test pilot has an accurately and recently calibrated airspeed indicator whereas a typical aeroplane may be out by as much as a couple of kts regardless of how accurately the pilot is flying. CASA’s flying tolerances for a short field landing are -1/+5 kts so an acceptable approach speed could easily be 7 kts faster than the test pilot so you’ve already increased the landing distance by 15% just because of that.
If you are flying a Super Decathlon and want to use a sensible approach speed of 1.3 Vs then you’ll be even faster. You may like to read this additional reference: USA FAA AC 91-79 Mitigating the Risks of a Runway Overrun Upon Landing. The effect of excess speed is explained on page 3 with a simple way of providing for it which you may like to use for the Super Decathlon. That AC considers other factors which push distances achieved by the average piklot way above the book figures.
CASA’s AC recommends a minimum safety factor of an extra 15% of distance however I prefer the higher factors of the UK CAA’s Safety Sense Leaflet 07, Aeroplane Performance.
I should mention that CASA still requires, per their Part 61 MOS, pilots undergoing a tailwheel endorsement in the Super Decathlon to use that approach speed of only 7% above the stall speed and achieve distances better than stated in the manual. That is without that 115% safety factor!
Finally, we may no longer use the declared density charts apparently as we must use the actual pressure altitude and temperature on the day. That’s a pity as the Super Decathlon provides airfield performance data up to very high density altitudes so would easily cover that very high ambient temperature at lower altitudes.