MFS said:
Is the nose landing gear oleo FULLY COMPRESSED under braking?..............................................
....then there will be an increase in AoA and corresponding increase in AoA and REDUCTION in load on mains due to increased aerodynamic lift.
I think that we have lost the physics bubble here. The four retardation scenario factors discussed (a-b-c-d above) are producing a strong nose-down pitching moment. By opposing that moment with progressive backstick soon after spoilers are up and reverse kicks in, we must end up with an effective weight increase on the main-gear - which helps the "rubber more intimately meet the road" so to speak - instead of glissading over the wet and slippery surface. It makes early braking much more effective by minimizing the intervention of anti-skid; and it's even got the improved directional control bonus. If you multiply that tyre/bitumen area interface increase on one tyre by all the tyres on a plane's main-gear, you can appreciate that the overall effect will be worthwhile.
<<<"then there will be an increase in AoA and corresponding increase in AoA and REDUCTION in load on mains due to increased aerodynamic lift.>>>
This is a little confusing but I imagine that you meant to say that
up elevator would cause the nose oleo to extend and consequently the wing's AoA to increase, thereby acting in an opposite sense to what is being claimed (i.e. reducing the weight upon the main-gear).
That that is a fallacy can be exposed by a simple illustration. Put a soft pea (or jelly-bean say) under the midpoint of a 12" rigid ruler and press down on both ends of the ruler. Do it with roughly equal force, simulating a pilot tempering his backstick input to just below what "could" (not saying "would") possibly cause the nose to rise. In other words, braking "for effect" by flying the attitude out the front window. The pea gets squashed of course. That simple experiment replicates the 4 retardation factors producing a nose-down moment at one end of the ruler and, at the other end, the opposing nose-up (i.e. tail-down) moment induced by the backstick. It adequately demonstrates that the resultant will be a down-force upon the maingear (the pea). Now do it with a new "soft pea" yet without the simulated pilot's up-elevator input. The nose drops, tail rises and the pea is unsquashed. That simulates the lack of anything other than the ruler's weight acting upon the "maingear" pea. It emphasizes the need for early introduced backstick braking when the chips are down (i.e. you suspect that you may have landed too far in on a slippery runway and urgently need max effective braking ASAP).
If I was in the RHseat and the reverse had cut in and I knew that we were
"down, come what may", yet running short of bitumen - well I'd be urgently calling for backstick and double-checking that the spoilers were up. I have a mind's eye image of both SWA 737 pilots urgently tromping their toe-brakes at Midway recently, the end looming large - yet salvation having been only a pole-grip away. That's the needless futility of not understanding the logic behind backstick braking.
Just to be clear: I recognise that OVERTALK and others are suggesting a progressive increase in tail download. The reasons I will CONTINUE to harp on about "full back stick" are:
(1) a procedure which requires modulation of input will ALWAYS carry the risk of someone pulling full back on the "if a little is good, a lot must be better" reasoning
(2) in order to obtain a significant improvement in braking a LOT of tail download is required - to the extent that full back stick will be used at some point
Disagree with your point (1) because I've been there and done that. Pilots that I have trained are told to introduce toe-braking first and then (but almost simultaneously) progressively feed in the backstick, ensuring that the nose stays down. Unless they soon ease up significantly on the brakes while maintaining "the pull", the nose won't rise - in part because, by that time, you will have slowed significantly. It is the hard braking that
keeps the nose down (as well as the airspeed loss). The danger of teaching it in a jet without maxarets or anti-skid on a wet runway is perversely that BLOGGs will overcook on the toe-brakes
without having adequate backstick in. It's the backstick that keeps the wheels loaded up and the tyres rotating. If you don't stop the wheel, you don't blow the tyre. I can recall writing that across the top of whiteboards in my briefing cubicle for many early type-conversion handling briefs. With autobrakes and anti-skid, the whole exercise is just too easy (unless you're ignorant of the technique and/or don't mind motoring mindlessly off the end). I reiterate that pilots are taught to use two controls at once (in fact directional rudder inputs plus toe-braking, aileron into wind and steering with cocked throttles to compensate for wind is a normal pilot activity that has four unique and discrete inputs). And if challenged, most could probably make an R/T call at the same time. You seem to have a picture of pilots as motor morons incapable of coping with a simple manipulative task. Landing a heavy-weight asymmetric airplane on a wet runway with 50 kts across used to make me salivate with anticipation..... not dread.
(2) <<<....to the extent that full back stick will be used at some point. Agreed (that full backstick may be used), but if that occurs more latterly in the piece, where's the harm? I reiterate that once heavy braking is underway, with or without reverse, that nose-rise just
won't happen.
There is already a nominal 67% planning margin between your "landing field length" and "actual landing distance". Unless a technique is going to make a significant impact relative to that 67% margin, it's not going to do much. That means a lot of redistribution of reactions, and that means a lot of tail load.
Unfortunately that 67% is roll-out, if I'm not mistaken. Few pilots manage to get to grips with the first few feet of runway. Indeed many PAPI's and glidepaths would have you landing over 1500ft in. When pilots misjudge and make a name for themselves, it's usually because they've
grossly misjudged. That can happen as a result of an optical illusion or down-sloping runway or a tail-wind. It's relatively easy to discard 2 or 3 thousand feet of bitumen, yet be unaware of it. Why? Because we haven't got eyes in the backs of our head and a touchdown too far in is not readily apparent. Distance-to-run marker boards are usually only found at military airfields. Landing 1/3rd into a runway at night off a low ceiling in poor vis? That's easily done. When the runway is additionally greasy and ATC has arranged a tailwind for you, don't start any wishful thinking after you've touched down and ripped it into reverse. At that point the writing's already on the wall at the far end of your marginal runway.