Anyone know the exact reason why aircraft weight makes little difference to the landing distance required? I would have thought the extra Kinetic energy stored in the aircraft would mean a longer required landing distance.

Perhaps you might tell us upon what data you base your statement ?

Comment might then be feasible.

Three principal considerations (putting runway conditions to one side for the moment) - weight (landing weight on the day), speed (approach speed will relate to stall speed which relates to weight) and ambient conditions (density height and wind).

Hi John. Thank you for your reply. Attached is the scan of the loading chart supplied in from Bob Tait in the supplements to his two RPL/PPL books.
Regards
Richard

The chart format is the old DCA P-chart with which I am quite familiar, having generated more than a few in years gone by.

Caveat - I would need either to see the flight test data used for the chart or run some simple tests on the aircraft make/model to check the figures to claim the following story as being categorically correct. However, it is the most likely tale and almost certainly what is going on here.

First, a bit of background.

Developing performance takeoff and landing charts is an expensive exercise due to the work which should go into doing it properly. The larger OEMs spend heaps and heaps of dollars on this work as getting the figures as low as allowable translates to better sales of the final product. The smaller OEMs, perhaps, invest a little less, accepting some additional level of conservatism .. it really is a balancing trick looking at certification costs against likely sales opportunities.

The DCA approach embodied in the chart you have posted utilised quite simple equations to generate the chart data from a comparatively small set of flight test data points. However, providing that the work was done with reasonable care, the resulting degree of conservatism was more than acceptable for the great majority of operations.

When it came to some importers, or aftermarket mod programs, folk sometimes would cut corners to save on the flight test expenditure and accept a significantly increasing level of conservatism .. often to the extent that the charts were an embarrassment..

One of the sometimes seen shortcuts with landing distance work was to use the MLW stall speed to generate a "one-size-fits-all" approach speed. What this meant was that, for low weights, the approach speed was considerably faster than what it could (should ?) have been. Now, think about your own flying .. what happens to the flare and subsequent air distance to touchdown if you approach considerably faster than usual ? .. right, it gets increasingly longer the more speed you are carrying above minimum.

Given that the techniques for some of these lower cost approaches to the task also hesitated to go for anything like maximum effort performance landing techniques, flare control and landing roll braking often would be somewhat inferior to what should have occurred. The end result was that one would see charts produced (and approved by DCA for the flight manual) which had the landing distance required INCREASING with DECREASING gross weight .. a tad strange but, in general, it didn't cause the operators too much problem so it kept cropping up from time to time.

You can tell that this (MLW stall related approach speed) occurred for this aircraft as the approach speed scheduled is a single value and that value will be for the MLW stall data (usually 1.3Vs .. keeping in mind that this stuff is figured in KCAS, not KIAS and then presented for pilot use as KIAS).

Now, depending on how diligent the pilot is during the tests considering flare/touchdown/braking aggressiveness the final effect on distance will depend on the balance between the increase in distance due to the higher approach speed as weight reduces and the reduction in distance due to getting the aircraft onto the ground as quickly as practicable and using heavier (to maximum) braking effort.

I suggest that, for this aircraft test series, the balance more or less evened things out, with the result that the agreement with DCA was that weight didn't alter things too much so that carpet could be left out of the chart.

The message to take away from this discussion is this - the result seen for this chart is NOT general and should ONLY be viewed as being applicable to THIS particular aircraft and THIS particular flight test data set. In general, a "better" run test program will see the usual reducing landing distance required as the landing weight reduces.

Thank you John for your most informative answer.

I've always understood that the statement on these P Charts that says landing distance is not affected by aircraft weight is based on the technique used in the flight tests that the charts are based on. As John says, this involves an approach at a particular IAS followed by a float and, after touchdown, maximum braking just short of skidding. Since extra weight results in firmer contact with the ground, the brakes are more effective for the heavy aircraft. The shorter ground run after touchdown due to more effective braking compensates for the increased kinetic energy due to increased weight.

However it's important to realize that this effect applies only to specific (very light weight) aircraft and is not applicable to all aircraft. It certainly does not apply to medium weight singles or multi engine GA aircraft.

Thanks Bob and John, I have a Lance so I will definitely be considering the weight effect on the landing roll given what you have posted.

Just to add a bit to Bob's comments ..

.. based on the technique used in the flight tests that the charts are based on.

Absolutely the case. However, it must be kept in mind that there is a range of approaches to the flying side of the work.

The professional TP employed by the OEM is very focused on getting the absolute minimum distance figures practicable. The flight test crew will scrub the runs with which they are not happy ... and then the aerodynamicists in the back room run the analyses and scrub a few more. The Regulator invigilates at the end for the certification assessment to make sure that the OEM doesn't push its luck too much.

Mind you, to get the good data involves sitting on a razor blade's edge at times ... and even the experts can have it go all pear-shaped on them .. one of the standard "Oh dear" moments cited was with a Douglas test point many years ago ...



I was in the aerodynamics group at Nomad in the 70s and can recall riding along on a performance landing test sequence with Stuart and Pat on one occasion ... Stuart had his faults but he certainly could make the girl sit up and play tricks like a trained dog ... it's not really "normal" flying like that with which you folks are familiar .. more a tad terrifying if you are hanging on in the back. That's why it is not sensible to use unfactored OEM data in line operations .. as you just are not going to achieve it .. hence the usual factors for AFM/POH use.

That's the reason why after market and such like work is done in a more sedate fashion, accepting a higher level of conservatism for the benefit of not putting the test aircraft at risk.

... an approach at a particular IAS followed by a float and, after touchdown, maximum braking just short of skidding.

The OEM TP will fly it on a bit like a carrier landing (not a whisker more than target speed - that's reason for scrubbing lots of tests) with minimum float and MAXIMUM braking as soon as the aircraft is on the ground and that braking effort will be maintained until the aircraft is stopped .. much lurching and so forth at that point.. The aftermarket test, in general, will be more relaxed ... but get nothing like results as good as the OEM flight test crew. However, aircraft risk and maintenance costs are far lower .. keep in mind that performance landing tests usually see a few sets of wheels, tyres and brakes scrapped .. even if the crew manage not to break anything else in the process

The shorter ground run after touchdown due to more effective braking compensates for the increased kinetic energy due to increased weight.

I'd probably debate that one .. for a first approximation, distance relates to energy relates to mass (call it weight for convenience) and speed squared .. speed is the real killer in stopping exercises. No different when you are driving a motor car. This is why any serious test card will be looking to run the approach speed in line with reducing stall speed as weight decreases.

Richard .. why don't you post a scan of the Lance's landing data ?