explain the "auto throttle" to me/us
Thread Starter
explain the "auto throttle" to me/us
From the Asiana threads, I am not sure how the "auto throttle" really works, or even the intent.
If the thing is set to hold an airspeed and change the engine thrust to hold that airspeed, then sounds like what we USAF pilots usually flew. You know, point the nose and move the throttles to hold speed. The nasal radiators did it different. Hold an AoA that gave you best approach speed for weight and such, then move throttles to control rate of descent. We SLUF ( A-7D) pilots found that the Navy procedure was great, but we still did a slight flare just before touchdown as we were not trying to catch a wire on the deck of a pitchin/rolling boat.
The Asiana crew appears to have used the procedure/technique we USAF dweebs used - point the jet, and move throttles or let HAL move them.
I would appreciate some explanations and rationale for different implementations.
If the thing is set to hold an airspeed and change the engine thrust to hold that airspeed, then sounds like what we USAF pilots usually flew. You know, point the nose and move the throttles to hold speed. The nasal radiators did it different. Hold an AoA that gave you best approach speed for weight and such, then move throttles to control rate of descent. We SLUF ( A-7D) pilots found that the Navy procedure was great, but we still did a slight flare just before touchdown as we were not trying to catch a wire on the deck of a pitchin/rolling boat.
The Asiana crew appears to have used the procedure/technique we USAF dweebs used - point the jet, and move throttles or let HAL move them.
I would appreciate some explanations and rationale for different implementations.
The Auto throttle does different things dependent on the mode it is in and the phase of flight.
As a general rule, though, I would suggest that in Airline flying, your "Point the jet use thrust to control speed" is what is normally used on approach.
As an example, the most usual approach in an Airliner would be an ILS- if done with the automatics, the AP would use pitch to follow the Glide Path, Roll to follow the Localiser, and the A/T would be in SPD mode, adjusting thrust to maintain the selected speed.
There are modes when this is not true, however- for instance if wishing to make a flight-idle descent, Flight Level Change mode simply reduces the thrust to idle, then uses pitch to maintain selected speed.
A pertinent factor about THAT mode is that after a few seconds at idle, the A/T mode changes to "Hold", which means it is basically dis-engaged and in standby. This allows manual changes to thrust to be made if you wish to vary your ROD.
In the Asiana case, it SEEMS that what the crew did was manually use pitch and roll to follow the approach path, EXPECTING the A/T to manage the speed, but the A/T was in fact in "Hold", and thus not doing so.
Hope that helps somewhat!!
As a general rule, though, I would suggest that in Airline flying, your "Point the jet use thrust to control speed" is what is normally used on approach.
As an example, the most usual approach in an Airliner would be an ILS- if done with the automatics, the AP would use pitch to follow the Glide Path, Roll to follow the Localiser, and the A/T would be in SPD mode, adjusting thrust to maintain the selected speed.
There are modes when this is not true, however- for instance if wishing to make a flight-idle descent, Flight Level Change mode simply reduces the thrust to idle, then uses pitch to maintain selected speed.
A pertinent factor about THAT mode is that after a few seconds at idle, the A/T mode changes to "Hold", which means it is basically dis-engaged and in standby. This allows manual changes to thrust to be made if you wish to vary your ROD.
In the Asiana case, it SEEMS that what the crew did was manually use pitch and roll to follow the approach path, EXPECTING the A/T to manage the speed, but the A/T was in fact in "Hold", and thus not doing so.
Hope that helps somewhat!!
Never understood why Nasals used the secondary effect of controls to fly approaches. 
When engaged, the AT moves the throttles as required to try to achieve the speed.
Only when the flight guidance system mode is in "Open Descent" (Airbii)/"Flight Level CHange"/Real Boeing/"Level Change"(others) does the AT generally go to Idle/Standby: in these modes the pitch position is used to control the speed (ala Nasals). The trap in this mode is, if hand-flying and ignoring the Flight Director (which probably is saying fly down) by pulling up the nose, the speed will reduce but the AT won't react. If the speed gets too slow because Bloggs continues to pull/hold the nose up, any decent AT system will spring to life regardless of previous pilot selections and save the day.
Apparently the 777 doesn't do this.
When engaged, the AT moves the throttles as required to try to achieve the speed.
Only when the flight guidance system mode is in "Open Descent" (Airbii)/"Flight Level CHange"/Real Boeing/"Level Change"(others) does the AT generally go to Idle/Standby: in these modes the pitch position is used to control the speed (ala Nasals). The trap in this mode is, if hand-flying and ignoring the Flight Director (which probably is saying fly down) by pulling up the nose, the speed will reduce but the AT won't react. If the speed gets too slow because Bloggs continues to pull/hold the nose up, any decent AT system will spring to life regardless of previous pilot selections and save the day.
Apparently the 777 doesn't do this.
Only when the flight guidance system mode is in "Open Descent" (Airbii)/"Flight Level CHange"/Real Boeing/"Level Change"(others) does the AT generally go to Idle/Standby:
The trap in this mode is, if hand-flying and ignoring the Flight Director (which probably is saying fly down) by pulling up the nose, the speed will reduce but the AT won't react.
If the speed gets too slow because Bloggs continues to pull/hold the nose up, any decent AT system will spring to life regardless of previous pilot selections and save the day.
Apparently the 777 doesn't do this.
Apparently the 777 doesn't do this.
Yes, I think it's a shortcoming.
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Interesting differences to the tiny boeing (aka 737). It reverts to ARM instead of HOLD and low speed will revert it to active, except below 27ft RA where it will revert to RETARD. THR HLD is only used during takeoff on that plane. But then, according to Capn Bloggs above it is not a "real" boeing as it uses Level Change and not Flight Level Change.
But then, according to Capn Bloggs above it is not a "real" boeing as it uses Level Change and not Flight Level Change.
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When going slower than the normal Ref+5 that an airliner would do, ROD is more effectively controlled with power, which is what the Navy does. Even I transition to that technique in my 185 when the necessity for going slow into a short strip is required. However, when increasing speed from the backside is necessary, then power comes to the rescue. Pitch only isn't enough.
In normal operations with an autopilot, the A/P controls pitch/glideslope and the autothrottles (if equipped) control power/airspeed, and of course, a combination of pitch and power is a blend for maintaining speed and profile.
In normal operations with an autopilot, the A/P controls pitch/glideslope and the autothrottles (if equipped) control power/airspeed, and of course, a combination of pitch and power is a blend for maintaining speed and profile.
Thread Starter
Yeah, Desert, guess we were used to a higher AoA than the big jets. Our Viper used about 13 degrees AoA, so we were close to the area of reverse command WRT power.
Of course, we trimmed like crazy to get to and then hold the AoA, then began using the throttle for precise touchdown point and rate of descent.
I still have trouble with all the throttle modes and such due to "connections" with the other HAL - the autopilot. And that's where my questions come from, as I only flew one jet with an Otto-coupled ILS mode and we had to use the throttle manually. And you better have had the sucker trimmed for best approach speed/AoA before "coupling". Figure 180 knots IAS basic, then more for gas/loadout.
It also looks like there's a plethora of implementations depending on what heavy you're flying.
Of course, we trimmed like crazy to get to and then hold the AoA, then began using the throttle for precise touchdown point and rate of descent.
I still have trouble with all the throttle modes and such due to "connections" with the other HAL - the autopilot. And that's where my questions come from, as I only flew one jet with an Otto-coupled ILS mode and we had to use the throttle manually. And you better have had the sucker trimmed for best approach speed/AoA before "coupling". Figure 180 knots IAS basic, then more for gas/loadout.
It also looks like there's a plethora of implementations depending on what heavy you're flying.
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And that's where my questions come from, as I only flew one jet with an Otto-coupled ILS mode and we had to use the throttle manually. And you better have had the sucker trimmed for best approach speed/AoA before "coupling". Figure 180 knots IAS basic, then more for gas/loadout.
You must be referring to the Mighty Voodoo, which I had the great pleasure of flying for 12 years/2,000+ hrs.
As a bit of a sidebar, I have considerable difficulty with the current culture/attitude regarding automation in general and auto-throttle use in particular. Acknowledging that I have never flown with an A/T, I find it very scary how it would appear that some folks on the flight deck these days seem to have an overwhelming reliance on this piece of automation.
For example, it would appear to me that in both this Asiana accident and the Turkish Airlines Flight 1951 in Amsterdam, the flight deck crew (three pilots in both cases) were under the impression that the A/T was handling the throttles, when in fact it was not, for different reasons. Regardless, in both cases, these six pilots did not recognize that the IAS dropped 40 KIAS below Vref, resulting in … guess what … a stall !
The problem that I have with the culture/attitude I referred to above is because, in my opinion at least, many pilots in this culture will say: “well, the faulty RADALT disconnected the A/T … or whatever … so it was not my fault”.
Wrong !
It is, it has been, and it always will be the pilots responsibility to ensure that the aircraft flies within its aerodynamic parameters. If the automatics aren’t doing the job, then the guy who signed for the aircraft has to take over. It's his responsibility, his job; it's why he is the aircraft captain !
I acknowledge that I am clearly a dinosaur as far as today’s automatics are concerned. But I am a pilot and I firmly believe that all pilots must be capable of keeping their aircraft flying.
Last edited by TLB; 13th Dec 2013 at 02:11.
Thread Starter
Yep, actual Dash-1 was 175 plus some knots for fuel over 3,000 pounds, but what the hell. Only the 104 had similar speeds on approach, best I recall.
Only had about 500 hours in the jet before the "war" called me, and I remained an attack puke next 16 years.
We sure appreciated the Cannucks flying 200 miles north of us toward the North Pole, but from our Grand Forks' orbit north of Gimli we could still see Baffin Bay, heh heh.
I am not a dinsosaur steeped in "hand flying" all the time, and I flew two of the latest and greatest jets with advanced avionics and nav features that the airlines did not get for years.
But I still have a problem understanding exactly how that auto-throttle fits in when merged with the auto pilot modes. I am confused. And I can't get a sim ride to see how it all works.
The folks on the AF447 thread know me, and they know my opinions regarding FBW, so we can skip all that stuff. They also know my opinion about a HUD, and I think a good HUD would have helped a lot for the Asiana pilot.
Only had about 500 hours in the jet before the "war" called me, and I remained an attack puke next 16 years.
We sure appreciated the Cannucks flying 200 miles north of us toward the North Pole, but from our Grand Forks' orbit north of Gimli we could still see Baffin Bay, heh heh.
I am not a dinsosaur steeped in "hand flying" all the time, and I flew two of the latest and greatest jets with advanced avionics and nav features that the airlines did not get for years.
But I still have a problem understanding exactly how that auto-throttle fits in when merged with the auto pilot modes. I am confused. And I can't get a sim ride to see how it all works.
The folks on the AF447 thread know me, and they know my opinions regarding FBW, so we can skip all that stuff. They also know my opinion about a HUD, and I think a good HUD would have helped a lot for the Asiana pilot.
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NY Times
Recent article quoted FAA as saying auto throttle will adjust power to maintain speed. Boing aircraft, the thrust lever actually moves when AT steps in to add power. Airbus, the throttle remains fixed.
The pilot said he noted the throttle position was not moving, but he thought the AT was going to step in and fix the problem.
No link found. Read it.
The pilot said he noted the throttle position was not moving, but he thought the AT was going to step in and fix the problem.
No link found. Read it.
Bottums Up
G'day Gums,
The venerable Douglas/Boeing 717, a DC9 on steroids, works thus:
Selecting Autoflight on the takeoff roll once symmetrical power increase is observed engages the Auto-throttles to the target takeoff power, be it REF power or flex/derate.
Passing the acceleration altitude entered into the FMS as part of the pre-departure procedures will see the take-off power reduce/increase to climb power, with the aircraft accelerating towards (usually) 250 kias. During this phase the AFS uses pitch to accelerate to or maintain 250 kias, and once through transition altitude to further increase to enroute climb speed. The Flight Mode Annunciator panel (FMA) will indicate SPEED on PITCH & T/O or CLIMB THRUST.
Once cruise level is reached the FMA will change to indicate SPEED (FMS or manually selected speed) on THRUST and LEVEL HOLD, and the auto throttle system will adjust power within the bounds of its capabilities to maintain the set speed.
When commencing a normal descent, the auto throttle system will for the most part continue to use thrust to manage speed, whereas pitch will manage the flight path. Should the aeroplane get higher than the computed flight path, the auto throttle will reduce power to idle, and may increase the IAS/MN to the maximum allowable minus a margin that is too small for comfort. On regaining profile the auto throttle will revert to SPEED on THRUST
A normally flown instrument approach or visual approach will see the SPEED on THRUST relationship remain, and in the flare the thrust will be reduced to idle, with reverse thrust being manually selected.
In any of the above modes, if the FMS detects the speed at Vmin - 5 kts, the auto throttle will advance the thrust, or if it had been turned off (but not broken) it will engage the auto throttle, and use a combination of thrust and pitch to maintain Vmin-5 and sacrifice altitude until the pilot intervenes and selects a faster speed.
In essence the 717 philosophy is to engage auto throttles during takeoff and leave them engaged until landing.
I think in my 8 years on type I've only had two occasions where the auto throttle system didn't work and both were a result of FADEC failures.
The venerable Douglas/Boeing 717, a DC9 on steroids, works thus:
Selecting Autoflight on the takeoff roll once symmetrical power increase is observed engages the Auto-throttles to the target takeoff power, be it REF power or flex/derate.
Passing the acceleration altitude entered into the FMS as part of the pre-departure procedures will see the take-off power reduce/increase to climb power, with the aircraft accelerating towards (usually) 250 kias. During this phase the AFS uses pitch to accelerate to or maintain 250 kias, and once through transition altitude to further increase to enroute climb speed. The Flight Mode Annunciator panel (FMA) will indicate SPEED on PITCH & T/O or CLIMB THRUST.
Once cruise level is reached the FMA will change to indicate SPEED (FMS or manually selected speed) on THRUST and LEVEL HOLD, and the auto throttle system will adjust power within the bounds of its capabilities to maintain the set speed.
When commencing a normal descent, the auto throttle system will for the most part continue to use thrust to manage speed, whereas pitch will manage the flight path. Should the aeroplane get higher than the computed flight path, the auto throttle will reduce power to idle, and may increase the IAS/MN to the maximum allowable minus a margin that is too small for comfort. On regaining profile the auto throttle will revert to SPEED on THRUST
A normally flown instrument approach or visual approach will see the SPEED on THRUST relationship remain, and in the flare the thrust will be reduced to idle, with reverse thrust being manually selected.
In any of the above modes, if the FMS detects the speed at Vmin - 5 kts, the auto throttle will advance the thrust, or if it had been turned off (but not broken) it will engage the auto throttle, and use a combination of thrust and pitch to maintain Vmin-5 and sacrifice altitude until the pilot intervenes and selects a faster speed.
In essence the 717 philosophy is to engage auto throttles during takeoff and leave them engaged until landing.
I think in my 8 years on type I've only had two occasions where the auto throttle system didn't work and both were a result of FADEC failures.
gums, tlb and all Fj- guys,
we had some kind of auto throttle as well, computed in our brain by reference to speed and attitude (present and desired) and implemented in our left elbow and hand.
Or did either of us need to do any kind of lengthy computing what kind of thrust setting would be suitable, aside from the first hours in the respective aircraft? How did we close formation takeoffs/ landings and WX penetration, how air refueling and AG bomb runs partly in single seat aircraft while navigating, comunicating and in bad days watching not to get shot down?
Now in the days of automation and magenta line training this ability to correlate speed and attitude to the thrust necessary has been lost, and a few (as i hope only a few) occupants of the front office ( i dare to name those pilots) even don't know anymore, what a AS indication is good for and what keeps an aircraft flying. Too many accidents happened that way.
we had some kind of auto throttle as well, computed in our brain by reference to speed and attitude (present and desired) and implemented in our left elbow and hand.
Or did either of us need to do any kind of lengthy computing what kind of thrust setting would be suitable, aside from the first hours in the respective aircraft? How did we close formation takeoffs/ landings and WX penetration, how air refueling and AG bomb runs partly in single seat aircraft while navigating, comunicating and in bad days watching not to get shot down?
Now in the days of automation and magenta line training this ability to correlate speed and attitude to the thrust necessary has been lost, and a few (as i hope only a few) occupants of the front office ( i dare to name those pilots) even don't know anymore, what a AS indication is good for and what keeps an aircraft flying. Too many accidents happened that way.
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Originally Posted by RetiredF4
we had some kind of auto throttle as well, computed in our brain by reference to speed and attitude (present and desired) and implemented in our left elbow and hand
Originally Posted by RetiredF4
Now in the days of automation and magenta line training this ability to correlate speed and attitude to the thrust necessary has been lost, and a few (as i hope only a few) occupants of the front office ( i dare to name those pilots) even don't know anymore, what a AS indication is good for and what keeps an aircraft fly
misbehaviour, individually or in crew. They call it their culture....
Originally Posted by gums
The folks on the AF447 thread know me, and they know my opinions regarding FBW, so we can skip all that stuff. They also know my opinion about a HUD, and I think a good HUD would have helped a lot for the Asiana pilot.
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Yes there is a tendency regarding the use of automation in "airline flying".
Save money in selection and training of pilots.
It is easier( = cheaper) to tell somebody to let the autopilot and autothrust do all the work, than to train him and to keep him proficient in manual flying.
Save money in selection and training of pilots.
It is easier( = cheaper) to tell somebody to let the autopilot and autothrust do all the work, than to train him and to keep him proficient in manual flying.
Thread Starter
I didn't want to hear that, LW.
I used otto a lot in my single-seaters, and even the family model I flew as a nugget. But it was simply a task management thing so I could pull out an approach plate for a strange field or figure out a new course or.....
My back seater in the VooDoo was never hesitant to ask me what the hell are you doing if I did something strange, heh heh. Same for my wingies in the SLUF and Viper.
I used otto a lot in my single-seaters, and even the family model I flew as a nugget. But it was simply a task management thing so I could pull out an approach plate for a strange field or figure out a new course or.....
My back seater in the VooDoo was never hesitant to ask me what the hell are you doing if I did something strange, heh heh. Same for my wingies in the SLUF and Viper.
Basic Boeing philosophy on the old types (737) - use AT with AP engaged. With AP disengaged, disconnect AT.
The automatics on the 737 are not top drawer though - the AT needs "help" in gusty conditions. The thrust levers move according to the AT demands.
The modes available for the 737 NG AT:
N1 - AT holds commanded N1 limit. Used in takeoff and climb, with aircraft pitching for speed
MCP SPD - AT holds target speed set on the MCP using thrust up to current N1 limit. Used in level flight, approach mode and in climb/descent when V/S is the active pitch mode
FMC SPD - AT holds target speed from the FMC - normally in cruise or in a path descent on a geometric descent path with VNAV PATH as the pitch mode
ARM - thrust levers static and able to be positioned manually. Normally idle thrust in descent with MCP SPD as the pitch mode (Level Change) Also provides very limited low speed protection.
GA - provides reduced thrust if the TOGA button is pressed once on a go around equivalent to around 2000fpm climb rate
It's a fairly simple system, with the main point being that if you are flying manually, you should be using manual thrust. I have never tried manual flight on approach with the AT in, and I imagine with the pitch couple it is fairly interesting.
The automatics on the 737 are not top drawer though - the AT needs "help" in gusty conditions. The thrust levers move according to the AT demands.
The modes available for the 737 NG AT:
N1 - AT holds commanded N1 limit. Used in takeoff and climb, with aircraft pitching for speed
MCP SPD - AT holds target speed set on the MCP using thrust up to current N1 limit. Used in level flight, approach mode and in climb/descent when V/S is the active pitch mode
FMC SPD - AT holds target speed from the FMC - normally in cruise or in a path descent on a geometric descent path with VNAV PATH as the pitch mode
ARM - thrust levers static and able to be positioned manually. Normally idle thrust in descent with MCP SPD as the pitch mode (Level Change) Also provides very limited low speed protection.
GA - provides reduced thrust if the TOGA button is pressed once on a go around equivalent to around 2000fpm climb rate
It's a fairly simple system, with the main point being that if you are flying manually, you should be using manual thrust. I have never tried manual flight on approach with the AT in, and I imagine with the pitch couple it is fairly interesting.
In the olden days (pre-FADEC), the autothrottles had a pretty important task in protecting engine ratings and keeping things like the 'climb' rating during the changing altitude/temperature during a climb.
With the advent of FADEC, that's no longer a significant issue - climb rating occurs at a constant throttle position, and it's pretty hard to have a significant overboost in Normal FADEC mode without really trying.
BTW, the autothrottle on all Boeing airplanes used to be more 'aggressive' in turbulence - it was intentionally slowed down for a number of reasons.
With the advent of FADEC, that's no longer a significant issue - climb rating occurs at a constant throttle position, and it's pretty hard to have a significant overboost in Normal FADEC mode without really trying.
BTW, the autothrottle on all Boeing airplanes used to be more 'aggressive' in turbulence - it was intentionally slowed down for a number of reasons.