I was recently told by a trainer in my airline that on the A320 if we hold the nose up after main gear touchdown then the flight control laws logic will memorise that pitch attitude after 5 seconds and will hold it there. Can anyone provide clarity on this? I have read the FCTM and know this is not a recommended procedure.
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2 Answers
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In flight mode the stick commands a load-factor. Which means it will be impossible to flare the aircraft, because as you pull on the stick, you'll be commanding a positive g-load.
Because of this the Airbus has a flare mode which activates at 50' RA. At 50' the pitch angle is stored (memorized). At 30' the aircraft commands a 2° nose down (it takes 8 seconds). The pilot would gently counter this by pulling back, resulting in a nice flare and no increase in the auto-thrust.
If the pilot did not float for way too long (they should not), the plane will touchdown before the 8 seconds pass, further eliminating the need to push forward on the stick to keep the nose gear planted.
The logic reverts to ground mode when two conditions are met: on ground for 5 seconds and pitch angle <2.5°.
If the 5 seconds pass and the nose is still +2.5°, the logic will remain in the flare mode and the THS will not reset to 0°. The pitch will revert to the one set at 50'. As the plane continues to slow down this may result in a nasty nose landing gear slam or a tail strike depending on the stored pitch angle and any gusts (kind of like this funny TV ad).
I do recommend you check your company's Flight Crew Operating Manual (FCOM) and to not use the internet for flight. If your instructor does not like too many 'why' questions during class, you can try and ask them after class. When you get your answers, try to validate them from the FCOM.
Some companies that fly extreme approaches have custom logic set by Airbus. So the installed logic may differ.
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$\begingroup$ This is pretty interesting. Do Boeing fly-by-wire aircraft like the 777/787 have similar logic for flare/ground detection? $\endgroup$ Commented Aug 27, 2017 at 18:37
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2$\begingroup$ @shortstheory - It's a good question. I think you should ask it on the site. Boeing calls it 'landing flare compensation'. But Boeing and Airbus use different fly-by-wire logics, so it's similar only in function, not in execution. A major difference is that Boeing uses an automatic thrust retard mode. $\endgroup$– user14897Commented Aug 27, 2017 at 19:10
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$\begingroup$ I agree that it is pretty interesting. I am baffled by the (apparently intentional) behavior relating to landing. Why would there be an easy mechanism to bash the nose gear/tail strike? Is there any overriding use for memorizing the 50' and 30' pitch angles? Why not let the pilot control the flare like on Cessnas? (Also, what is
FCOM?) $\endgroup$– wallykCommented Aug 28, 2017 at 5:23 -
5$\begingroup$ @Neil_UK Given the huge number of A320s in service (7,600 have been built, including A318/9/21) and the number of landings they successfully perform every day, I would infer that the controls are pretty intuitive. In particular, the activation of flare mode causes pulling back on the stick to raise the nose (as I believe one would expect) and, after landing, one would naturally want to get the nose wheels on the ground. So, although the description sounds complicated, it seems that the software makes the stick behave exactly as the pilot would expect, even if they didn't know the aircraft. $\endgroup$ Commented Aug 28, 2017 at 12:34
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1$\begingroup$ @ymb1, the biggest difference is that Boeing emulates mechanical controls within the flight envelope, while Airbus uses the flight laws to auto-trim. The automatic thrust-retard mode is rather a small detail compared to that. $\endgroup$ Commented Jun 10, 2018 at 19:09
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I do not think what your instructor said is true. The flare mode activates at 50 ft radio altimeter height, where auto trim is cancelled and the pitch attitude memorized by the Elevator Aileron computers (ELAC). At 30 ft, the memorized attitude is used as a reference to push the nose down by 2 degrees over a period of 8 seconds. The flare law is necessary to give the pilots a conventional flare experience. Because Airbus aircraft are pitch stable, if there is not a flare law, every time you pitch up to flare, that pitch attitude will be maintained and there would not be the normal sink rate (visual aid to commence the flare) you see when you try to land a non fly by wire aircraft. The flare mode changes over to ground mode, when the aircraft pitch attitude is less than 2.5 degrees and the aircraft is sensed on the ground for more than 5 seconds.
A prolonged nose up attitude would not make the computers memorize any pitch attitude. The flare mode is done with memorizing attitude at 50 ft and there is not a nose down pitch applied by the computers either, as it is done with it from the point aircraft passes 30 ft and the 8 second count down is over. The only thing that will happen is that the aircraft will not enter the ground mode, but will remain in flare mode and the trimmable horizontal stabilizer (THS) would not reset to zero. As flare mode is a direct stick to elevator relationship, the elevator moves with the corresponding stick demand. So, if you pull back on the stick, the elevator will move up and the pitch attitude will increase depending on the aircraft energy level.
The aircraft FCTM recommends the pilots to smoothly fly the nose wheel onto the runway. It says fly, not drop. After the main wheels touch the ground, you should try to hold the nose wheel and slowly make it touch the runway. I have seen the nose tilted up for more than 5 seconds and have done it myself as well, because if you let go off the nose too quickly, it tends to smack the ground quite firm. In these situations, the aircraft always behaved normally as you expect it to. But yes, it important not to over do it, as it can lead to a tail strike. That is the only danger associated with it. So, you should properly modulate your pitch up inputs to hold the nose off after landing.
A320 FCTM
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$\begingroup$ Hi Anas. RE "As flare mode is a direct stick to elevator relationship...": I'm afraid that's not correct; in flare mode the stick deflection commands a pitch angle, as explained by Airbus here (PDF; airbus.com). $\endgroup$– user14897Commented Feb 3, 2021 at 13:50
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$\begingroup$ @ymb1 In my airline's A320 manual which is provided by Airbus states: Flare mode is essentially a direct stick-to-elevator relationship (with some damping provided by the load factor and the pitch rate feedbacks). $\endgroup$ Commented Feb 3, 2021 at 15:42
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$\begingroup$ Even in a conventional aircraft, with a direct stick to control surface relationship, you are essentially commanding a pitch attitude in the longitudinal axis. So, what was written in the safety first article is not incorrect. It is just worded differently. Safety first articles states: 'A pitch sidestick deflection corresponds to a commanded pitch attitude.' $\endgroup$ Commented Feb 3, 2021 at 15:44
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$\begingroup$ The updated law (second graph) shows extra logic based on pitch rate. However, going by the basic FCOM explanation, I don't see your point (or how our answers differ) when you stated, "I do not think flare law works like that." If the flare mode is not switched, and there's no more artificial nose down, and the THS doesn't reset, it can lead to a tail strike / NLG slam. $\endgroup$– user14897Commented Feb 3, 2021 at 16:05
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$\begingroup$ Why would it slam down. I have been flying the A320 for quite some time now. I have kept the nose up after landing for more than 5 seconds and I have never noticed a change in aircraft behavior. How do you think normal airplanes land then? Their THS does not automatically reset. Does it lead to a tail strike? The pilot has to hold off the nose to prevent it from slamming. The flare in Airbus aircraft is very very conventional. $\endgroup$ Commented Feb 3, 2021 at 16:23