What's the point of POH performance figures?

+1 for the figures here, I have found them to be pretty accurate (that’s mainly C172/C172RG/C182/C182RG). What is really important, is to read the full description of the technique used if needed for a ‘performance’ t/o or landing as it can vary between very similar aircraft types. As example: the C172 (at least the variants I have flown, there are so many…) stipulates flaps 10deg for short-field t/o whereas the C172RG has zero flap for same.

I think my TB20 runway perf data is accurate (that graph was in the CAA approved POH but the FAA one had just tables…) but the POH contains some cruise data called “best economy” and “best power” and these terms are not explained anywhere.

The POH is dated 1988 – a maneuover by Socata to minimise recertification of the GT version – which predates engine instrumentation, and it is thought the former is peak EGT and the latter is obviously something rich of there, but questions to Socata were never answered.

As a result the book figures for range are easily improved on by about 20%, while pilots who don’t believe in too much leaning (a significant % of the participants in their mostly-US-based user forum) continue burning some 20% too much fuel One of them accused me of having an illegally gas-flowed engine… but two c. 1984 TB20s which I have flown in (which have the instrumentation too) do exactly the same figures as mine!

And I think this may be fairly common – achieving much better enroute performance than the book says.

performance numbers are usually correct but as already mentioned, make sure you read notes section carefully. Sometimes you´ll find out the cruise figures for weight of 2600 lbs but not for 3000 as MTOW (SR20, doc number P/N 11934-003) – difference is not big but still. Sometimes landing distances are enhanced by using low approach – Tecnam 2008 declaring 54 kts on short final. On 54 kts the P2008 is almost stalling, leaving aside fact that 54 kts is by the same POH declared as “Optimal touchdown speed: 54 KIAS” on page 4-16. So you are probably expected just to hit the ground, don´t attempt to flare.

Peter wrote:

illegally gas-flowed engine

What does that mean?

It’s bollox. Gas flowing (cleaning up the inlet manifold) makes almost no difference to these engines. It is sometimes done in the US Exp market but the extra HP on the dyno is barely measurable, apparently. Whether it is legal on a certified engine in a certified aircraft is a matter on which opinions differ; some say it is legal if you merely remove casting flaws i.e. the dimensions remain within the manufacturer’s spec.

I fly a Diamond DA40 with upgraded engine (Centurion 2.0s).
The PoH is based on the “old” Thielert 1.7 engine.

Since the engine upgrade results in a performance improvement, there is no need to update the PoH.

So I have the funny situation that the aircraft performs much better than the manual says, but I’m legally restricted to the limits as stated in the PoH.

I agree with Peter and the others posting here. I do read a lot of POH’s and also have been busy doing performance models for different flight planners a lot. I would think that mostly the figures are ok. BUT, and that is a very big BUT: Be very careful how to read derived figures.

One figure I never even look at is range calculations, but I do my own from TFD and TAS/FF. Some of these range calcs are dangerously overstated and, frankly, PR but not useable.

With a new plane I fly I usually try to verify the figures within the first couple of flights and keep the flight plan verification going over the time.

With the Mooney, which has one of the worst POH’s I’ve ever come across (for this ’65 model) I found that TAS/FF are quite correct, so is the rest of the (very few and incomplete) performance figures. I had to compile Time/Fuel/Distance tables out of those data as well as some other stuff which simply is not there. Wet runway (grass) for instance, airfields above 5000 ft, and so on.

With my old Cessna, I found the figures pretty accurate once we revised the prop to it’s original specs. However, range figures were total SciFi, how they were calculated is beyond me.

The only way to do this is really to verify the data in flight for your particular airplane. You can start with some basic stuff like FF and TAS(if you have a TAS Display, otherwise use 4 way GPS) for a few reference levels and exact power settings. If those figures are correct, then there is a good chance the rest is as well.

I love playing with performance and POH’s and am notorious in creating huge Excel orgies with it. My fun but one gets to know the airplanes capabilities pretty well after that. Also when I still worked with my own flightplanning system (which was capable of handling anything from a J3 to a 747) putting in a GA plane would lead to quite a lot of head scratching but also valuable insights. POH’s can outdo Agatha Christie in terms of entertainment if you’re into it

The certification requirements for presentation of the performance data have evolved over the decades. A “CAR 3” plane (generally older than 1980, but some newer planes are still CAR 3, has only modest requirements for presenting performance data. A newer “Part 23” aircraft will have much more detailed requirements for presentation of performance data, the newer, the more data.

Required performance data will be presented in an “approved” Flight Manual. That data will be very accurate in all cases. My experience has been that all Cessna data is very accurate, no matter how old (right back into the ’50’s). Not so much for Piper. Piper had a bad habit of overlaying the very basic approved flight manual with a second, very flashy “Pilots Handbook”. For my experience, some of these pilot’s handbooks had extremely optimistic performance data, which came right out of the sales department, not the flight test department, or FAA. The result is that there is not a hope those performance numbers can be achieved. The sales department did not care, you’d already bought the plane by the time you figured that out! I found this first while performance flight testing a Twin Comanche for a propeller change. I could not come close to meeting the “P’sH” climb numbers, but all the pilots who flew the modified Twin Comanche said it climbed much better. It turns out it did, but still nowhere near the P’sH figures. I found buried deep, an FAA approved flight manual for the aircraft, which did not list any climb performance (not required back then).

So, if you’re referring to an “approved” flight manual for performance, take it to the bank. BY the way, the standard Cessna “Information Manual”, “Pilot’s Operating…” etc, will tell you inside the front cover exactly what it’s status is in terms of being FAA approved, and to what standard, but in any case, it’ll be right anyway….

Very interesting. I always thougt that “marketing writes the POH” was a myth, clearly it wasn’t some time in the past.

Contrary to aonothe popilar myth, Part 23 requires the performance figures to be achievable by the average pilot, not a superstar test pilot, which probably contributes to their realism post the marketing brochure era.