B777 - FMA Modes: THR vs SPD
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B777 - FMA Modes: THR vs SPD
Hello,
I have some questions that I would appreciate the answers to.
1) In flight school, we are taught that pitch controls speed and thrust controls flight path angle. For example, if one is slightly below the GS, in order to get back on the GS, the theory is that one would first increase thrust in order to regain the vertical flight path. Then, this would require a compensatory increase in pitch in order to maintain the speed. Is this how it is done in airliners as well? Or are airliners flown with pitch for flight path and thrust for speed since their engines take longer to spool up? I recently talked to a real-life A330 pilot who told me that airliners are flown like Cessnas - IE pitch for speed and thrust for flight path.
2) I am confused as to the difference between THR and SPD on the 777. If airliners also inherently follow the "pitch for speed, thrust for flight path" philosophy, does that mean that in SPD mode on the 777, the autopilot still uses pitch for speed and thrust for flight path? For example, assume that one is flying in V/S mode at -1000 FPM, speed 250 knots. Assume that one wants to increase speed to 270 knots. In order to accomplish this, does the plane first pitch down to accelerate to 270 knots, and then increase its thrust to maintain the v/s at -1000? Or does the autopilot use thrust to accelerate to 270 knots and reduce pitch to maintain the -1000 FPM?
3) In the 777 FCOM, it says this:
"THR – autothrottle applies thrust to maintain the climb/descent rate required by the pitch mode ".
So if I got it right, in a climb, when FLCH is selected on the MCP, the MCP speed window opens at the current
airspeed and the aircraft’s speed is still controlled through the elevators but the FMA pitch
mode changes to FLCH SPD. The FMC considers the MCP altitude setting and commands
the autothrottle to provide thrust (THR) as required to achieve the resulting altitude change in
two minutes (120 seconds).
If a climb is commanded that exceeds the ability of the autothrottle to achieve the change in
two minutes the thrust goes to the current thrust reference maximum setting.
If the airliners like the 777 are indeed flown like a GA plane (i.e. pitch for speed and thrust for flight path), then wouldn't this mean that there is essentially no difference between the THR mode and the SPD mode? In that case, what exactly is the difference between THR and SPD then?
4) Finally, what is the difference between ATHR on Airbus and AT on Boeing? I heard they worked quite differently.
Any answer would be highly appreciated.
Thanks a lot.
I have some questions that I would appreciate the answers to.
1) In flight school, we are taught that pitch controls speed and thrust controls flight path angle. For example, if one is slightly below the GS, in order to get back on the GS, the theory is that one would first increase thrust in order to regain the vertical flight path. Then, this would require a compensatory increase in pitch in order to maintain the speed. Is this how it is done in airliners as well? Or are airliners flown with pitch for flight path and thrust for speed since their engines take longer to spool up? I recently talked to a real-life A330 pilot who told me that airliners are flown like Cessnas - IE pitch for speed and thrust for flight path.
2) I am confused as to the difference between THR and SPD on the 777. If airliners also inherently follow the "pitch for speed, thrust for flight path" philosophy, does that mean that in SPD mode on the 777, the autopilot still uses pitch for speed and thrust for flight path? For example, assume that one is flying in V/S mode at -1000 FPM, speed 250 knots. Assume that one wants to increase speed to 270 knots. In order to accomplish this, does the plane first pitch down to accelerate to 270 knots, and then increase its thrust to maintain the v/s at -1000? Or does the autopilot use thrust to accelerate to 270 knots and reduce pitch to maintain the -1000 FPM?
3) In the 777 FCOM, it says this:
"THR – autothrottle applies thrust to maintain the climb/descent rate required by the pitch mode ".
So if I got it right, in a climb, when FLCH is selected on the MCP, the MCP speed window opens at the current
airspeed and the aircraft’s speed is still controlled through the elevators but the FMA pitch
mode changes to FLCH SPD. The FMC considers the MCP altitude setting and commands
the autothrottle to provide thrust (THR) as required to achieve the resulting altitude change in
two minutes (120 seconds).
If a climb is commanded that exceeds the ability of the autothrottle to achieve the change in
two minutes the thrust goes to the current thrust reference maximum setting.
If the airliners like the 777 are indeed flown like a GA plane (i.e. pitch for speed and thrust for flight path), then wouldn't this mean that there is essentially no difference between the THR mode and the SPD mode? In that case, what exactly is the difference between THR and SPD then?
4) Finally, what is the difference between ATHR on Airbus and AT on Boeing? I heard they worked quite differently.
Any answer would be highly appreciated.
Thanks a lot.
In flight school, we are taught that pitch controls speed and thrust controls flight path angle. For example, if one is slightly below the GS, in order to get back on the GS, the theory is that one would first increase thrust in order to regain the vertical flight path. Then, this would require a compensatory increase in pitch in order to maintain the speed. Is this how it is done in airliners as well? Or are airliners flown with pitch for flight path and thrust for speed since their engines take longer to spool up? I recently talked to a real-life A330 pilot who told me that airliners are flown like Cessnas - IE pitch for speed and thrust for flight path.
Just fly like the autopilot does on the ILS. If it gets below slope, it pulls the stick back to get back on the glideslope then corrects with thrust if it gets slow. If the speeds increases, the autothrottles pull back to slow back down. It doesn't pull the nose up,off the glideslope, to slow down! Think of the primary effects of controls. Then it will be obvious how to fly. We don't use the secondary effects of controls in jets because the reaction takes too long (unless you fly a 737...
).
If the airliners like the 777 are indeed flown like a GA plane (i.e. pitch for speed and thrust for flight path),
I don't fly the 777 but my machine does a similar thing if only climbing a couple of thousand feet ie lower thrust than full climb thrust. However, I wouldn't use this particular unique scenario to generalise. For a prolonged climb, FLCH will set and hold climb thrust, and the pitch controls the speed. Similar for an Idle/FLCH descent; throttles put to Idle by the autothrottle and stay there (unless moved, but that is another story), pitch controls the speed.
For example, assume that one is flying in V/S mode at -1000 FPM, speed 250 knots. Assume that one wants to increase speed to 270 knots. In order to accomplish this, does the plane first pitch down to accelerate to 270 knots, and then increase its thrust to maintain the v/s at -1000? Or does the autopilot use thrust to accelerate to 270 knots and reduce pitch to maintain the -1000 FPM?
"THR – autothrottle applies thrust to maintain the climb/descent rate required by the pitch mode ".
You're probably an FO in the right seat. Just remember the old adage when flying an approach: "Always remember and forever take heed, right hand for flightpath and and left hand for speed!"
what is the difference between ATHR on Airbus and AT on Boeing? I heard they worked quite differently.
Knuckleheads who flew onto carriers need not reply!
Now I'll get my hat and coat!
Last edited by Capn Bloggs; 25th Mar 2020 at 07:11. Reason: Fixed the quotes
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Basically the autopilot is a machine working on code. It's a simple device really.
As said above, pitch for flightpath and thrust for speed control. if you're low pitch up. it's far easier for a machine to understand so it's done that way.
As to modes. For Boeings, you will either be climbing level or descending. A LVL CHG command is the simplest, it will either give you climb thrust or idle and then pitch to maintain speed. VNAV climbs or descents are similar except the computer is being clever and switches from speed to mach and protects the envelope by changing speed at 10,000 or whatever is programmed into the FMC.
In V/S, again, it's simple. Pitch to give the correct VS and thrust to maintain speed as selected.
An Airbus is physically not much different. The programs and controls look different but achieve the same results.
Where you can get confused is the throttles- a Boeing will use the Autothrottle to move the thrust lever and the thrust lever tells the EEC to give you the required thrust.
An Airbus lever is effectively a thrust limiter. It will just use it know the limits you've selected. As far as I can figure out anyway.....
As said above, pitch for flightpath and thrust for speed control. if you're low pitch up. it's far easier for a machine to understand so it's done that way.
As to modes. For Boeings, you will either be climbing level or descending. A LVL CHG command is the simplest, it will either give you climb thrust or idle and then pitch to maintain speed. VNAV climbs or descents are similar except the computer is being clever and switches from speed to mach and protects the envelope by changing speed at 10,000 or whatever is programmed into the FMC.
In V/S, again, it's simple. Pitch to give the correct VS and thrust to maintain speed as selected.
An Airbus is physically not much different. The programs and controls look different but achieve the same results.
Where you can get confused is the throttles- a Boeing will use the Autothrottle to move the thrust lever and the thrust lever tells the EEC to give you the required thrust.
An Airbus lever is effectively a thrust limiter. It will just use it know the limits you've selected. As far as I can figure out anyway.....
Only half a speed-brake
because on final approach, where you will be most likely to handfly, he's wrong. On final approach, control speed with thrust, and pitch with the control column. Jets are not Cessnas. Actually the correct way to fly a Cessna is the same;
Indeed, for a B737 (the global largest jet fleet) it is the easier way, due to strong pitch/power couple and non-augmented flight controls. I have no opinion on other types which would be relevant to this particular thread.
Only half a speed-brake
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The age old discussion. You’ll use a combination of pitch and power to get the vertical speed and airspeed that you want.
Now I’m no A330 pilot, but from my understanding of the control laws, using power to maintain a vertical rate on final would make for an interesting ride down the glideslope.
Now I’m no A330 pilot, but from my understanding of the control laws, using power to maintain a vertical rate on final would make for an interesting ride down the glideslope.
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There are whole world's regions where the opposite is preached, in line with the original post. And it has served them well for generations.
Indeed, for a B737 (the global largest jet fleet) it is the easier way, due to strong pitch/power couple and non-augmented flight controls. I have no opinion on other types which would be relevant to this particular thread.
Indeed, for a B737 (the global largest jet fleet) it is the easier way, due to strong pitch/power couple and non-augmented flight controls. I have no opinion on other types which would be relevant to this particular thread.
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What I have found as I’ve moved into bigger, faster, and more slippery machines is that in a jet on approach, a small change in pitch causes a moderate change in flight path but a only a small change in speed, therefore pitching for speed doesn’t work very well. Point it where you want it to go and adjust thrust for speed makes the most sense to me! On the other hand, in a C172 a small change in pitch only causes a small change in flight path and causes a moderate change in speed, so pitching for speed in something slow and draggy works if you prefer to think of it like that. The truth is, as said up-thread, it is the combination of pitch and thrust that gives you your performance and if you change one then you will probably have to change the other as well.
"Power for altitude, pitch for speed" is a good basic rule for newbie pilots.
It tends to avoid stalls if inexperienced people are taught to not simply "pull the UP lever (the yoke)" for climbs, but add power. And generally it is the way aerodynamics work anyway - reduce or increase power without a trim change, and most fixed-wing aircraft will tend to nose down and descend, or nose up and climb, all by themselves. (High thrust-line aircraft sometimes only after a brief excursion in the other direction).
And without a power change, pitch will certainly change the speed - sometimes dangerously.
However, the dynamics of tracking a glideslope, with a heavy high-inertia aircraft, with slow-spoolup turbines, on a constant descent angle, generally work more responsively with small corrections being done with "pitch for altitude, power for speed." If you are 400 feet AGL and below glideslope, you want to GO UP RIGHT NOW - faster that turbines/thrust/power can respond. Pitch responds faster.
It is just an adjustment in habit that is part of that advanced training.
It tends to avoid stalls if inexperienced people are taught to not simply "pull the UP lever (the yoke)" for climbs, but add power. And generally it is the way aerodynamics work anyway - reduce or increase power without a trim change, and most fixed-wing aircraft will tend to nose down and descend, or nose up and climb, all by themselves. (High thrust-line aircraft sometimes only after a brief excursion in the other direction).
And without a power change, pitch will certainly change the speed - sometimes dangerously.
However, the dynamics of tracking a glideslope, with a heavy high-inertia aircraft, with slow-spoolup turbines, on a constant descent angle, generally work more responsively with small corrections being done with "pitch for altitude, power for speed." If you are 400 feet AGL and below glideslope, you want to GO UP RIGHT NOW - faster that turbines/thrust/power can respond. Pitch responds faster.
It is just an adjustment in habit that is part of that advanced training.
Especially on a FBW aircraft like the 777 where adding power does nothing to the pitch: you would have to wait for a change in speed before seeing a pitch change as trim takes effect. Rather messy if you’re trying to follow a glide slope and possibly worse than that for a low-level go-around...
Only half a speed-brake
Since the stoning has ceased, perhaps it's good courtesy to return some pieces for another round of throws:
Airbus FBW has a reactive auto-trim, hence there always is displacement due to pitch/power couple. Can't be flown hands-off although the deviation is small. I'm also told that for landing at 100' RA that feature stops completely for the A330.
I think the passengers of AF447 wish they had a pilot who would pitch for speed first, instead of firewalling the thrustlevers. A330.
I think the owners of EK521 wish they trained the pilot to use thrust forward to climb and vice-versa. B777.
There are many non-jet A/C where there is no pitch-power couple, ATR to name one. Using power to manoeuvre in pitch is pointless, and insisting not only silly but just would not work. On those, yes.
On a machine with a decent power-pitch couple, using pitch (elevator) for V/S and only then thurst for speed is sloppy. But also very easy as the PFD tells you exactly that. Flight director command first, and later the speed trend arrow. CoM?
Airbus FBW has a reactive auto-trim, hence there always is displacement due to pitch/power couple. Can't be flown hands-off although the deviation is small. I'm also told that for landing at 100' RA that feature stops completely for the A330.
I think the passengers of AF447 wish they had a pilot who would pitch for speed first, instead of firewalling the thrustlevers. A330.
I think the owners of EK521 wish they trained the pilot to use thrust forward to climb and vice-versa. B777.
There are many non-jet A/C where there is no pitch-power couple, ATR to name one. Using power to manoeuvre in pitch is pointless, and insisting not only silly but just would not work. On those, yes.
On a machine with a decent power-pitch couple, using pitch (elevator) for V/S and only then thurst for speed is sloppy. But also very easy as the PFD tells you exactly that. Flight director command first, and later the speed trend arrow. CoM?
Last edited by FlightDetent; 26th Mar 2020 at 23:12.
Coincidentally, posted on AvHerald on 24 March:
”An Alliance Airlines Fokker 100 on behalf of Virgin Australia, registration VH-UQN performing flight VA-1251 from Brisbane,QL to Rockhampton,QL (Australia), was on final approach to Rockhampton's runway 33 when the aircraft encountered moderate turbulence at about 300 feet AGL, which caused the loss of airspeed. While attempting to arrest the loss of airspeed, the airspeed fell below the minimum approach speed because the flight crew failed to push the throttles forward.”
”An Alliance Airlines Fokker 100 on behalf of Virgin Australia, registration VH-UQN performing flight VA-1251 from Brisbane,QL to Rockhampton,QL (Australia), was on final approach to Rockhampton's runway 33 when the aircraft encountered moderate turbulence at about 300 feet AGL, which caused the loss of airspeed. While attempting to arrest the loss of airspeed, the airspeed fell below the minimum approach speed because the flight crew failed to push the throttles forward.”