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Seneca V - Accelerate Stop Distance

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paul_sherry

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Although based in Europe and have EASA licences, I also fly some N reg aircraft over here and decided to add my AMEL to my FAA PPL and IR. Taking a flight test usually prompts me to get my head in the books and do a bit of background reading. And one of the things I did was to re-read the performance section of the POH for our Piper Seneca V.

This question to the forum concerns Accelerate Stop Distance (ASD) and I have scanned in a copy of the ASD performance chart for our Seneca and attached it as a PDF file.

The weird thing is that (according to the chart) as the aircraft gets heavier, the ASD gets shorter....

Example - Sea Level Airport, ISA day (15 deg C), no wind. Let's assume a level paved runway.

ASD @ 4750lb - 2530ft
ASD @ 3500lb - 4100ft

So, with an aircraft 1250lb lighter, the ASD is 1570ft (approx 475m) LONGER...

I just simply cannot rationalise this outcome. The aircraft has 440HP (2 x 220HP TCM TSIO360RB engines). That's a fixed thrust. Acceleration = Force/Mass so if the mass is 36% more then the acceleration must be 36% less. Which means that the aircraft simply must take longer, in both time and distance, to reach the defined abort speed of 81 knots. I accept that the acceleration won't be constant up to the abort speed because of varying prop efficiency, but that's a fixed variable in both scenarios as the thrust won't change with aircraft weight - just the acceleration.

Then take the stopping distance....

The same brakes and wheels have to dissipate 36% more energy at the higher weight than the lower weight (from the same speed of 81 knots). Energy = 1/2 x mass x velocity squared. The velocity (81 KIAS) is the same so energy is directly proportional to weight. Again, from simple physical laws, the braking distance of a heavier object, giving the same retarding force, will be greater.

The only possible explanation that I can think of is that the braking (retarding) force is higher in the higher weight aircraft (more weight on wheels) than the lower weight aircraft. That's certainly possible (even likely) but in no way can it explain a reduction in ASD by 1570ft, especially as the acceleration distance to 81 knots simply must be longer (Newton's 2nd law).

I posted this on one of the European GA forums and we have been debating this for a few days and it's gone to 4 pages. But no-one has come up with a sensible explanation (except that Piper have made a mistake and put the weights in the wrong way round...)

Can anyone here do any better?

Thanks

Paul

View attachment SenecaV-AccelerateStopDistance.pdf
 













 
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