landing distance and technique for jet certification

[captainng]

hi guys
having a discussion with a collegue about how the landing distance is worked out with regards to flap, thrust and speeds over threshold. for the 737 800 on certification with the test pilots
my questions to you are

(1) at what point are the thrust levers closed or idle thrust assumed for landing distance calculation?

(2) at what speed over the threshold ie 50 feet is assumed speed(ie vref)?

(3) are there any margins built into the certification data or are they exact figures (ie thrust idle, speed+-,height above threshold?

any references greatly appreciated as there is a wager involved

not putting my thoughts in as not to bias the answers!!!

[tdracer]

I can answer one part of your question:

To obtain stopping distances, they perform 'max performance' landings under near ideal conditions - basically plant the airplane on the runway and then stand on the brakes (no thrust reversers) - and measure the stopping distance. That's done at various weights and landing speeds.

A factor is applied to the resultant stopping distances (my memory says 1.6 but I'm not sure about that) to obtain the cert stopping distances.

Of course in the real world you get things like wet/contaminated runways and less than perfect landing techniques - with some compensation provided by use of the reversers. The factor they apply is intended to account for that.

[underfire]

Depending on the aircraft, class configuation (169 or 189, or custom) and configuration, ie CAT C or CAT D your airline has certified the 800 for, will determine many of the varibles you are asking about.

Vspeeds, Vref and flap schedules are based on weight of the aircraft. Flap speeds are updated from the FMC and are based on Vref40 +and additive based on flap setting.

Idle thrust point is variable, especially if you are on a coded procedure.

Not sure what you mean by ht above threshold...are you talking wheel ht?

[john_tullamarine]

With the usual caveats, start with the relevant frozen Design Standard and ACs.

Background reading in how to do it might usefully start with

(a) FAR 25 (http://www.ecfr.gov/cgi-bin/text-idx?SID=af648bde6a5e9522d4cc83772eecf612&tpl=/ecfrbrowse/Title14/14cfr25_main_02.tpl)

(b) AC 25-7C (http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%2025-7C%20.pdf).

(Noting that your aircraft of particular interest may have used now-superseded data in which case you need to dig around for that older data rather than use the current data above).

Generally, any OEM is out to get the absolute best results feasible during the test program. Dud runs will be discarded, in turn, by the FT crew, and then the performance aerodynamicists back at the factory. A bit more argy bargy will ensue with the particular Certification folk to end up with an agreed basis for the AFM.

(a) idle thrust selection will be appropriate to the aircraft's handling characteristics and one should refer to the flight procedure in the AFM to get an idea of what might have transpired.

(b) the 50ft point is worked out post flight test by analysing the FT data records

(c) actual test speeds will be as close as the FT crew can get them to the nominated Vref. Final data tweaking post test will refine the data for any minor errors. Generally, tests are done in optimum weather and the TP generally nails the numbers pretty accurately.

(d) test analysis will be to get basic raw data referenced back to the desired numbers

(e) AFM landing data will be according to the regulatory requirements. The AFM will disclose what the specific data basis is for the Type and Model.

(f) operational landing data will use factored data - normally 1.67 for normal operations. This is the pilot's basic protection and please accept that the line pilot out in the real world isn't going to be able to reproduce the test data. The raw data should be viewed as the absolute minimum brick wall ..

I suggest that it's best not to mix up certification and operational data and requirements - two different animals.

[captainng]

thanks for the replies they have been helpful!!!
i will check the AFM to see what thrust procedure to be used.

[AirRabbit]

I sincerely hope that this doesn't sound too terribly arrogant ... but it was just a while ago (yeah, I know ... relevant term) that I posted one of my traditionally "longer" posts ... but it was specifically on this issue. Feel free to skip it if you choose ... no hard feelings.


I know just how much everyone on this forum likes to have someone come along and “tell them” how it’s to be done. So, for those of you here who are older than ½ of my age – stop reading and go on down to the next post. OK, maybe you don’t know my age, but I was around when dirt was invented – that should give you an approximation. But, for you folks who are less than ½ of my age, listen up – this may be important to you, and, after a little practice, you just may be able to teach those guys who have now stopped reading how to really land an airplane. This procedure is applicable to every airplane from a C-152 to a B-747 (I haven’t flown a B-52 or the A-380, but I’ll bet it works there, too) – it also works in calm conditions, head winds, tail winds, cross winds, CAVU conditions, snow, ice, rain, simple IMC conditions, and even FLIR-aided IMC conditions.

As almost all of the folks above have indicated – the last portion of the final approach should be flown in the configuration in which you plan to land, and flown at a constant speed of 1.3 Vs (computed in that configuration), plus ½ of the steady state wind (not to exceed an additive of 20 knots) plus all of the gust factor. I personally believe that this steady-state condition should be established at 1000 feet AGL, but I know that some operations allow this altitude to be lower – but in the passenger revenue world I’m not aware of any that are below 500 feet AGL.

You should cross the runway threshold at what ever is the minimum threshold crossing height – for most transport category airplanes this should be about 50 feet. And at that point you should have been able to bleed off the airspeed additives you’ve been holding for steady-state wind (only the steady-state wind additives) – you’ll still have the 1.3 Vs plus all the gust factor. This will require you to continue to fly the airplane to the runway. Some operators recommend that you begin to reduce power at this point – if that is the procedure you’ve been taught, fine – but keep the airspeed constant until you begin the flare (that may mean pushing the nose over a bit – hopefully it will only require nose down pressure and not nose down movement. The point to which you should be flying at this point (the “aim” point – that point that doesn’t move up or down in the windscreen) is a point on the runway surface about 2/3 of the way between the threshold and the fixed distance markers (for the C-150 guys, this aim point should be the numbers themselves and for the B-747 guys, the aim point should be the fixed distance markers or just beyond).

OK, now for the flare. The question that always comes up is, “what attitude do I flare to?” When you start to flare is critical. You will want to reach your flare attitude with the main wheels something between 1 and 5 feet from the runway surface (1 foot or so for the C-152 guys and 5 feet for the B-747 guys … yes, I know how difficult it is to imagine the mains at 5 feet above the runway from the B-747 cockpit – but remember, you’re good at your job! – Make it 5 feet!) The change in the attitude from when you initiate the flare to reaching the flare attitude should take just about 3 seconds (no less than 2 for you C-150 guys and no more than 4 for you B-747 guys) and you should wind up with the main wheels “just off the runway surface. The speed you should have when you reach the flare attitude should be just below what you carried from the threshold to this point – between 5 and 15 knots – the smaller number for the smaller airplanes and the larger number for the larger airplanes. The attitude should be just exactly what it would take to maintain level flight from this point all the way down the runway. What I’d have you practice would be, “do not climb, do not descend, do not accelerate, do not decelerate; we’ll go around at the end of the runway.” I’d also have you mentally locate the position on the belly of the airplane exactly between the main gear (the body gear for you B-747 guys) and I’d tell you to fly down the runway (no climb, no descent, no faster, no slower) with that point on the belly of the airplane exactly over the runway centerline – and to do that with whatever crab angle you need to do it. Of course you’d have to add a bit of power – since you had the throttles back but this is OK for practice.

I’d have you do this exercise as many times as was necessary to get you comfortable with when to initiate the flare, how quickly to flare, and to what attitude you need to stop the flare with the main gear just off the runway surface. The key here, getting you to recognize when to start the flare and how quickly to flare, is to get you to recognize what attitude to reach at the end of the flare – THAT attitude is the LEVEL FLIGHT ATTITUDE.

Once you’ve got it, as you begin the flare you begin the throttle reduction. The idea is to get the throttles to the idle position as the mains touch the runway. As you pull the throttles back, you will notice the nose getting heavier – don’t let it move down. Increase the back pressure on the elevator controls – not to move the nose up – rather to just keep it from moving down. Over that 3 seconds, the airspeed continues to decelerate, while the airplane continues to descend, going from just above the runway to ON the runway. Level Flight Attitude is the attitude from which you want to land the airplane. Your touchdown should be firm but not hard, the kinetic energy of the airplane should be moving in the right direction, the nose should be able to be flown to the runway rather quickly as it is not unnecessarily high to arrest a high sink rate. You should be over the center of the runway, with the controls already properly positioned for the landing run.

If you had been carrying a crab angle to counter a crosswind, the crab should be removed in exactly the same time as the flare takes – 3 seconds. The pressure applied to the rudder pedal to pressure the nose around to line up with the centerline of the runway should start with the back pressure on the control column to flare. As you probably know, this may take some “into-the-wind” aileron to counter the tendency of the forward sweeping wing to rise … but, unless the wind is quite strong, you won’t be in the air long enough to have the wind blow you downwind off the centerline. Of course, if the wind IS quite strong, you may have to add a bit more aileron to slightly (very slightly) dip the wing tip in the up-wind direction.

I offer just one caveat. If you discuss this with your chief pilot or fleet captain and they absolutely forbid you to fly and land this way – pay attention to your company and forget what I’ve said. This is not an attempt to thwart the way your company procedures require you to operate. If this is different from the way you normally approach and land, I do not recommend that you do it without everyone in the cockpit knowing what you are going to do, no matter what position you are flying – if you can swing it, I’d recommend practicing it in the simulator with someone who knows what they are doing. I think you’ll be surprised at how easy this becomes, and how consistent your landings will become as well – night, day, rain, snow, clear, no matter. Consistent landings are good things to cultivate. Also, if you try this and just simply think it is the epitome of wrong-headedness, let me know and I’ll buy you a beer. However, if you think it is the correct way to land, let me know and I’ll buy you two!


The techniques described above, when followed even roughly, will allow anyone to "land" the airplane they are flying well within the calculated runway required distances given gross weight and landing configurations.

[safetypee]

The principles for determining landing distance (UKCAA perspective) are in AIC 14/2006 (www.theairlinepilots.com/forumarchive/pilotslounge/landingperformanceoflargetransportairplanes.pdf).
Although most modern aircraft are certificated with the ‘arbitrary’ method, see CS25/FAR25, the original 737 might have been certificated with the ‘reference’ method and grandfather rights claimed for all variants thereafter.

The current European requirements are in CS 25.125 (http://easa.europa.eu/agency-measures/docs/certification-specifications/CS-25/CS-25%20Amendment%2011.pdf) (~page 25). FAR 25.125 is similar, if not identical.

Change in thrust and other information is in AMC 25.125 (~page 267):- “No changes in configuration, addition of thrust, or nose depression should be made after reaching 15m (50 ft) height.” For most aircraft this probably means idle thrust from 50ft.

Operational requirements EU (IR)-OPS / FAR 121, require the certificated distances (AFM) to be factored for dispatch according to runway/weather conditions.
IR-OPS (http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2012:296:0001:0148:EN:PDF) – check this ref.

Operational requirements for in-flight pre-landing assessment of the ‘required’ landing distance are open to various interpretations and any factoring is usually based on un-factored manufactures data (QRH) which may differ from the certificated distances (read the QRH small print). There there are differences in the requirements for assessment and factoring between EU and FAR.
Furthermore, most manufacturers are now implementing ‘new style’ Operational Landing Distances (OLD) which provide more realistic data for a range of conditions; this may be published (QRH) with factors as 'FOLD'. http://sapilot.com/a320/otherfile/In_flight_landing_performance.pdf