Landing with AT vs without AT engaged

[markjj 47]

Hi,  a while ago (right after the release of the PMDG 777), I posted a topic concerning the final approach behavior of the 777, wondering if there was some sort of flare assist.  Well, it turns out that there is something very much like what I described, but I wouldn't exactly call it flare assist.  To test this, I encourage you to manually fly an approach and landing without the auto thrust, and then do the same thing again, only with auto thrust engaged this time (like Boeing and most airlines recommend).  There is a stunning difference in the last 30 or 25 feet AGL.  Basically, up until a few days ago, I ALWAYS flew with auto thrust off, as I found it to be a little sluggish to maintain my speed, especially in windy conditions.  Anyways, now I ONLY fly with it engaged.  Not only because of its great ability to manage speed (I never knew it was this good), but because of the fact that the aircraft becomes MUCH easier to handle during the final moments of flight.  Without AT engaged, you'll find that the 777 is very insensitive to yoke inputs during the last 30 feet of flight.  Basically, it was near impossible to make any noticeable pitch changes during the last 10 feet without applying a large amount of yoke input.  But after doing tons of landings with AT engaged, I noticed that the aircraft becomes much more sensitive in the flare when AT is engaged.  This is not my imagination...try it yourself if you haven't already.  To notice the difference even more, you must try making pitch changes about 20-5 feet above the ground in both scenarios (one with AT, one without AT).  You will notice that the aircraft is very forgiving and that you can easily make changes to the pitch attitude right before touchdown versus not having AT on.  Anyways, feel free to comment on what you guys think of the difference, and it'd be interesting to know what Boeing refers to this behavior as.  Thanks
-Mark Javornik

[ybarshavit 0]

Interesting, I've felt that the AT on flare was very slow to react and always kept the aircraft in the air due to ground effect. I found if I retard the levers to idle at 10 ft I get a really nice gentle touch on the runway.

[3-2-1-Now 126]

Very interesting finding. I thought I noticed a difference as well, but figured there was something in the way I was flying (when using manual thrust, I retard the throttles earlier and faster than the AT system).

 

I was going to fly a few circuits anyway - I'll give this a try.

 

Best regards,

Robin.

[3-2-1-Now 126]

Hi,

 

It seems that because the AT system retards the thrust to idle later, and a bit slower than manual thrust, it is this fact that results in the different feel of the flare. I have found that the FBW seems to be trying so hard to compensate for the thrust change, that the stick input becomes secondary, with the apparent feeling that it is being ignored unless you pull harder than you think you should. I found that I had to pull much harder than with the AT system engaged.

 

On the second manual landing attempt, I retarded the thrust levers much slower, and found a better pitch reaction.

 

I also experimented with the FBW and "attacked" it by performing hard pitch up/down maneuvers to extreme angles (without stalling/overspeed) and found that the FBW seems to have some interesting behaviors.

 

I also discovered that the FBW will not allow you to deflect the elevator to its maximum deflection despite having the stick pulled all the way back. This resulted in the curious behavior of the flight control page showing the elevator at half deflection for some time, if it reacted at all.

 

At one point, when close to VMO with the TRS set to 180 kts, I couldn't push at all, until I centered the stick, waited a moment, then tried it again. I also discovered during the pitch up part of the tests, that the FBW will limit the max g/pitch rate initially, then it will suddenly release the protections, and result in an accelerated pitch rate. It seemed to be related to passing a certain pitch angle, I would guess around +30 degrees. It was also approaching VMO at this point (but not exceeding it). I think it was during these rapid pitch rate changes that resulted in exceeding maximum g load. It was consistent in its behavior, and required that you pitch up to maybe 45-50 degrees to see it.

 

It seems that the FBW elevator commands are being used to replicate the "speed stability" to the detriment of pilot input (it seems the FBW overrides pilot input, even if no limits are being exceeded and no protections are in effect).

 

I also found that the FBW system will result in the maximum commanded deflection being offset by the deflection that it had when the stick was centered. e.g. if it had 1/4 nose down elevator at neutral stick (e.g. to counter engine acceleration), then full aft stick would result in 3/4 elevator deflection. Note that it was at very low speed when I was observing this behavior, but not stalled or in the speed protection range.

 

It would seem there is a small issue with mixing of commands.

 

I also managed to over g the aircraft, and break multiple systems (worth trying if you want to use the QRH!). This didn't appear to affect the tests - the FBW mode was "Normal".

 

Best regards,

Robin.

[lauderj 0]

Interesting topic. Has anyone attempted to recreate the approach of Asiana 214 into SFO based on the sequence of events described in the recently released NTSB accident report? During the last mile or so of the approach, while still a tab high on the glide path, the pilot was descending on VS below the missed approach alt of 3000 ft. The altitude window was set at 3000 and the pilot switched to flight level change and the airplane began to climb, which of course would be normal behavior. The pilot immediately disconnected autopilot ( but not auto throttle) and manually retarded the thrust levers to idle. This put autothrottle to hold mode and the airspeed decayed to below stall without the autothrottle reengaging in speed mode. The aircraft hit the sea wall.

 

When I try to recreate this sequence of events, my autothrottle will always reengaging to SPD mode once speed decays below was is dialed in the speed window.

 

Has anyone been able to recreate what happened in Asiana 214?

 

Jerry Lauderbaugh

Avid PMDG pilot

[tcas_climb 45]

Hi,

 

It seems that because the AT system retards the thrust to idle later, and a bit slower than manual thrust, it is this fact that results in the different feel of the flare. I have found that the FBW seems to be trying so hard to compensate for the thrust change, that the stick input becomes secondary, with the apparent feeling that it is being ignored unless you pull harder than you think you should. I found that I had to pull much harder than with the AT system engaged.

 

On the second manual landing attempt, I retarded the thrust levers much slower, and found a better pitch reaction.

 

I also experimented with the FBW and "attacked" it by performing hard pitch up/down maneuvers to extreme angles (without stalling/overspeed) and found that the FBW seems to have some interesting behaviors.

 

I also discovered that the FBW will not allow you to deflect the elevator to its maximum deflection despite having the stick pulled all the way back. This resulted in the curious behavior of the flight control page showing the elevator at half deflection for some time, if it reacted at all.

 

At one point, when close to VMO with the TRS set to 180 kts, I couldn't push at all, until I centered the stick, waited a moment, then tried it again. I also discovered during the pitch up part of the tests, that the FBW will limit the max g/pitch rate initially, then it will suddenly release the protections, and result in an accelerated pitch rate. It seemed to be related to passing a certain pitch angle, I would guess around +30 degrees. It was also approaching VMO at this point (but not exceeding it). I think it was during these rapid pitch rate changes that resulted in exceeding maximum g load. It was consistent in its behavior, and required that you pitch up to maybe 45-50 degrees to see it.

 

It seems that the FBW elevator commands are being used to replicate the "speed stability" to the detriment of pilot input (it seems the FBW overrides pilot input, even if no limits are being exceeded and no protections are in effect).

 

I also found that the FBW system will result in the maximum commanded deflection being offset by the deflection that it had when the stick was centered. e.g. if it had 1/4 nose down elevator at neutral stick (e.g. to counter engine acceleration), then full aft stick would result in 3/4 elevator deflection. Note that it was at very low speed when I was observing this behavior, but not stalled or in the speed protection range.

 

It would seem there is a small issue with mixing of commands.

 

I also managed to over g the aircraft, and break multiple systems (worth trying if you want to use the QRH!). This didn't appear to affect the tests - the FBW mode was "Normal".

 

Best regards,

Robin.

Do you think that maybe some of this behavior could be as a result of PMDG attempt to simulate the loading/artificial feel on the control column during heavy loading, mixed with their simulation of the FBW?

[ybarshavit 0]

Interesting topic. Has anyone attempted to recreate the approach of Asiana 214 into SFO based on the sequence of events described in the recently released NTSB accident report? During the last mile or so of the approach, while still a tab high on the glide path, the pilot was descending on VS below the missed approach alt of 3000 ft. The altitude window was set at 3000 and the pilot switched to flight level change and the airplane began to climb, which of course would be normal behavior. The pilot immediately disconnected autopilot ( but not auto throttle) and manually retarded the thrust levers to idle. This put autothrottle to hold mode and the airspeed decayed to below stall without the autothrottle reengaging in speed mode. The aircraft hit the sea wall.

 

When I try to recreate this sequence of events, my autothrottle will always reengaging to SPD mode once speed decays below was is dialed in the speed window.

 

Has anyone been able to recreate what happened in Asiana 214?

 

Jerry Lauderbaugh

Avid PMDG pilot

Interesting, I've never thought about this before. But the fact you are not able to recreate this is puzzling. Either the NTSB is wrong, or part of the simulation in PMDG is wrong. I'm going to try and create this in the weekend, see if I can crash her.

[markjj 47]

Thanks for the interesting replies guys.  Haha I always thought that the explanation to the aircraft's sensitive behavior during flare was just something Boeing put in to give pilots some extra authority during the flare and to make the plane a bit easier to control, but I never thought that it went any deeper than that.  I'll look around to see if there is a solid answer, but again thanks for the replies and I'm sure there's a reason behind the behavior.

[Michael Moe 494]

Interesting topic. Has anyone attempted to recreate the approach of Asiana 214 into SFO based on the sequence of events described in the recently released NTSB accident report? During the last mile or so of the approach, while still a tab high on the glide path, the pilot was descending on VS below the missed approach alt of 3000 ft. The altitude window was set at 3000 and the pilot switched to flight level change and the airplane began to climb, which of course would be normal behavior. The pilot immediately disconnected autopilot ( but not auto throttle) and manually retarded the thrust levers to idle. This put autothrottle to hold mode and the airspeed decayed to below stall without the autothrottle reengaging in speed mode. The aircraft hit the sea wall.

 

When I try to recreate this sequence of events, my autothrottle will always reengaging to SPD mode once speed decays below was is dialed in the speed window.

 

Has anyone been able to recreate what happened in Asiana 214?

 

Jerry Lauderbaugh

Avid PMDG pilot

 

Interesting, So the pilot did an actual error using FLCH ? right ?

Why would he touch the throttle at all ?

 

Should he not have been looking at the speedtape and instead just pull the yoke forward ? AT was engaged

 

Michael

[Michael Moe 494]

I am trying to replicate but wonder it there is A/T protection during CLB ? Thought i would practice around 10000 feet first.

 

Scenario is as described .

 

FLCH and F/D off on PF side . A/P off, Retract throttles to idle (have "always" overide in FMC)

 

But A/T commands FLCHSPD (THR) or ALT (SPD) (not HOLD) all the time

 

Anyone else tried this scenario?

 

Michael 

[Michael Moe 494]

Again , 

 

Trying to replicate with no success ,

 

what is going on during my landing here ? ( i know the F20 settings)

 

I am all over my brakes to stop the plane :-) Throttle is in idle but A/T is green and G/A is lighted up as i had hit the button(which i did not)

 

[ger 2]

Hi ,

 

before SP1 i allways landed with AT off , because i couldnt use reverse thrust  when the main wheels were on the ground and the nose wheel stil above ground  with AT on , it seemed like the engines were too slow returning to idle al landing with AT on , when i switched AT off i could idle the engines 30 feet above ground and when the main gear touched down my engines were fully idle  and  i immediately could use reverse thrust as i saw often at Schiphol when a 777 landed , they use reverse thrust immediately when the mains are on the ground with the nose wheel staying in the air for a long time  before the go down ,  i can't do that with AT on and still can't with SP1 , with AT off i can .

 

Best regards ,

 

Gérard

[Michael Moe 494]

Hi ,

 

before SP1 i allways landed with AT off , because i couldnt use reverse thrust  when the main wheels were on the ground and the nose wheel stil above ground  with AT on , it seemed like the engines were too slow returning to idle al landing with AT on , when i switched AT off i could idle the engines 30 feet above ground and when the main gear touched down my engines were fully idle  and  i immediately could use reverse thrust as i saw often at Schiphol when a 777 landed , they use reverse thrust immediately when the mains are on the ground with the nose wheel staying in the air for a long time  before the go down ,  i can't do that with AT on and still can't with SP1 , with AT off i can .

 

Best regards ,

 

Gérard

+1 same here

 

Michael

[3-2-1-Now 126]

 

 

Do you think that maybe some of this behavior could be as a result of PMDG attempt to simulate the loading/artificial feel on the control column during heavy loading, mixed with their simulation of the FBW?

 

I would doubt it. Also, PMDG don't need to simulate the feel unit because our controllers are always wanting to return to center, which by default gives a feel of weight.

 

Wait for SP1b and try it again.

 

Best regards,

Robin.