While I have been aware for several years of the difference between Boeing’s certification and advisory data, I have just in the last few days been reading all of your posts both in this thread and in the previous thread referenced by Leadsled. This has, I hope, brought me sufficiently up to date with the output of TALPA and so forth. Now the question is, how to implement the new FAA landing distance assessment requirements with the data provided by my company for my airplane, which is an MD80.
As I indicated in my previous post, it appears to me that a great deal of the old regulatory margin, intended to protect against average risk, has been disposed of in favor of more precise identification of several specific risks such as runway slope and additional speed additives, etc. I understand that the regulatory requirements for dispatch remain. What I am trying to do is make sure I understand what is going on with the implementation of the TALPA output.
To better understand this, I have attempted to dissect the numbers in my performance manual. I’d like to float an example here and see if anyone with a better understanding of this can provide a course correction to my thinking.
At 130,000 lbs, my MD80 has a Vref of 132 KIAS. The dry certification landing distance, at sea level, etc., is 5030 feet. I can remove the regulatory factor from this by dividing by 1.67, which yields 3011 feet as a raw, flight test landing distance.
Just taking an educated guess at what the air distance might have been during this certification, I cranked through the equation in AC 25-7A for a calculated air distance at 132 KIAS, and that yielded 1121 feet. I have no idea whether that is the actual air distance associated with the actual flight test data, but it should serve as a reasonable assumption.
If we go back to my dry distance of 5030 feet, I could, in theory, be dispatched to a 5100-foot runway. This runway would have a touchdown zone of one third the runway length, or 1700 feet. Policy says that I must touchdown within that touchdown zone or go around.
If I fly this approach exactly as a test pilot would, with the single exception being that instead of an 1121 foot air distance, I use the entire touchdown zone, I end up using an additional 579 feet. Consequently, my flight-test-quality experiment gives me an actual landing distance of 3590 feet. This is still well within the LDA; the 1.67 factor has accommodated my float across the touchdown zone pretty well.
Now I went back and tried this same process with the advisory data that I have in my performance manual. Same weight, same Vref, but I selected the first column of “FOLD” data…we don’t yet call it that, but that is what I assume it is. So, for good-to-medium braking action, I get a FOLD of 7620 feet.
Next, I remove the factor, in this case dividing by 1.15. This gives 6626 feet.
In order to arrive at an estimated air distance for this condition, I simply assumed that I would spend 7 seconds at 132 KIAS before touching down, which yields 1559 feet.
Next, I assumed a limiting runway for this condition, which would theoretically be 7700 feet. One third of this distance yields a touchdown zone of 2566 feet. The difference between the estimated air distance and the full length of the touchdown zone is 1007 feet. If I add this to the unfactored OLD of 6626 feet, I get a landing distance if 7633 feet…67 feet short of the end.
If I repeat the same experiment with the straight “medium” braking action data, I end up 14 feet past the end of the limiting length runway of 8100 feet.
So it seems to me that a policy-legal landing at the far end of the touchdown zone uses up the entire 15% margin without anything else going wrong. The operational variables cited in AC 121-195(d)-1a include “piloting technique” and “flight path deviations” as part of the reasoning for a 67% dry margin. Yet in all of the reading I’ve done lately, including SAFO 06012 and AC 91-79A, I see a lot of discussion regarding the use of deceleration devices, etc…but no discussion regarding piloting technique per se, outside of an enormous get-out-jail-free card for the authorities that mentioned operator training and experience.
Indeed, it seems to me that, assuming a runway-limited situation, that the test-pilot precision that we all agree is not representative of line operations is exactly what will be required to make these new numbers work.
What am I missing?