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In reading the reports and Sully's comments regarding the flight that was ditched in the famous events of US Airways Flight 1549, I am wondering how the Airbus control system works to flare an airplane under normal flight conditions.

Sully stated that since the automated system did not let him flare as much as he wanted to, he was unable to reduce vertical speed further.

Is it that under normal flight, the aircraft has forward speed to generate lift, and the descent rate is thus less? I thought (I'm not a pilot) that it was desirable to basically stall by flaring at the last moment in normal powered landing, thus sticking it to the runway for traction, wheel spin-up, etc.

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  • $\begingroup$ Related: Why is it not recommended to hold the nose up on the A320 after touchdown? -- the post explains how the A320 flare works, although I think you are asking how does the flare differ from jet-liners to light planes, if so, please clarify it, thanks :) $\endgroup$ – ymb1 Jun 7 '18 at 15:22
  • $\begingroup$ He would not have wanted to flare into a stall. At stall the lift drops dramatically and would increase vertical speed. It wouldn’t let him flare all the way to the point before the stall. $\endgroup$ – TomMcW Jun 7 '18 at 19:03
  • $\begingroup$ He was "mushing" towards the water and was interested in hitting the water aft first but the flight computer would not allow him raise the nose just before impact. $\endgroup$ – jwzumwalt Jun 8 '18 at 2:23
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    $\begingroup$ @TomMcW, not necessarily. Stall is not always abrupt, and in some circumstances one may want to use up all the margin up to the actual stall. Even if you go slightly beyond stall (top of the lift curve), but you are just a few feet off the water, vertical speed will not have time to develop significantly, yet every moment you are winning time, i.e. reducing forward speed. $\endgroup$ – Zeus Jun 8 '18 at 2:37
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    $\begingroup$ tough crowd over here at Aviation. Guess I'll go watch Top Gun and cry... $\endgroup$ – MarsJarsGuitars-n-Chars Jun 9 '18 at 17:56
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You don't do full stall landings in airliners. You arrive over the threshold at reference speed (the full flaps approach speed), ease the thrust off (how soon depends on the airplane, but you need to be at idle before touchdown) and start to flare but only enough to reduce the sink rate to close to zero and then you tease the pitch to maintain a gentle sink until you touch. If you wanted to do a full stall type landing, there is sufficient energy still available at reference speed that you would have to hold it off and float halfway down the runway, and you'd touch down with the stick shaker going. Not a desirable outcome in a 150,000 lb soda can.

If you arrived over the threshold too much below reference speed, there is no energy reserve to save yourself if you are sinking too fast and there goes your tires, if you're lucky.

Water landings often don't go so well if the tail is too low (if the tail touches in too nose high an attitude, it tends to bounce upward and drive the nose over and things can go south). So it's possible the FBW system actually protected them by limiting Sully's ability to pitch up. They seemed to have touched down at an ideal attitude, getting a nice ski effect from the nacelles without the tail hitting too early.

In the famous Airbus airshow incident, the airplane descended gently into the trees as it struggled along way on the back side of the power curve (in that regime, the only option is to descend). Had the crew more pitch up authority available to them, it is possible they would have semi-stalled into the trees at a much higher sink rate. That was Airbus's response to the criticism over the FBW's pitch limiting behaviour.

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  • $\begingroup$ Strictly speaking, descending is not the only option on the back of the power curve. You just increase thrust, if available. Most agile aircraft will push through stall with ease. So, hypothetically, if you need to win time to spin up the turbines, it may make sense to go beyond the protected angle. Or, maybe it's better to go down with higher vertical speed but lower forward speed: still, less energy. It'd rather leave this decision to the pilot. $\endgroup$ – Zeus Jun 8 '18 at 2:46
  • $\begingroup$ The Airbus's pitch attitude was 15 degrees at the time they applied TOGA, so the wing with takeoff flap would have been on the edge of a stall. There was no pitch margin left and the only thing keeping them in the air was ground effect. Key bit: if it was a conventional airliner with a stick shaker and pusher, what would have happened is the stick shaker would've been going as they mushed along, then as they kept pulling, the pusher would have fired and they would've nosed into the ground instead of settling in a steady state. So yes I think the computers saved their asses. $\endgroup$ – John K Jun 8 '18 at 14:44
  • $\begingroup$ (but you need to be at idle before touchdown) Not in the MD-80 haha. At heavy weights with lots of airplane nose up "ANU" trim with the direct cable elevators if you pulled power to idle over the runway and she started to sink you would pull back on the elevator only for nothing to happen! Most left the power in and then swiped it at touchdown to avoid this. Only jet I've ever known where landing "power on" was encouraged. $\endgroup$ – S80driver Aug 6 at 14:20
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Below 50ft radio altitude Airbus flight controls go to Flare mode. Key change there is that vertical control (stick front/back) changes from normal pitch rate close to conventional elevator movement.
This means that while in normal flight, ie. cruise, holding the stick back at a certain position you would see constant nose-up movement on a given rate. On flare mode you are able to control (and maintain) pitch attitude more precisely, as the stick input will result in elevator deflection rather than pitch rate.
Flare mode isn't available after loss of radio altimeters.
Also after a dual engine failure, the aircraft would have most likely been in Direct law, meaning that all automatic trims are lost. This might impact elevator efficiency, since the horizontal stabilizer would not automatically move.

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