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B737 Mis-set T/O Power at BRS - 4 Mar 2024

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B737 Mis-set T/O Power at BRS - 4 Mar 2024

Old 6th Jun 2024, 08:49
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Originally Posted by EXDAC
Well, now I'm curious. In your experience what autothrottle systems do not drive all the thrust levers through a single clutch pack? The MD-11 with FADEC controlled engines drives all three levers through a single clutch pack. The main difference for FADEC control is that the thrust levers send electrical signals representing lever position to the FADEC instead of actuating mechanical cables that drive the fuel control valves. FADEC, or pre-FADEC, thrust control systems typically include a limited authority trim system that compensates for throttle stagger.
My licenses did not include Auto flight systems, MD-11 or any Airbus types, but I understand, more recently, that each throttle lever can de-clutch, so that a Pilot can move them individually; on the B737-NG and B777 each thrust lever is moved with an independent autothrottle servo motor (ASM); no doubt other model Boeing's have similar. I believe two clutch/motor packs are used much after the design of the one B727 unit.
Open to ridicule!
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Old 6th Jun 2024, 13:43
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Originally Posted by aeromech3
I understand, more recently, that each throttle lever can de-clutch, so that a Pilot can move them individually;
All the autothrottle systems that I am familiar with had drive clutches that allowed each lever to be moved independently by the crew whether autothrottle was engaged or not. Holding or moving a single lever did not stop the servo driving the other lever(s). DC-10 worked like that so this is not a new innovation.

Last edited by EXDAC; 6th Jun 2024 at 16:28.
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Old 6th Jun 2024, 17:11
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Slight thread drift but relating to thrust control: In 1998 I wrote to a Boeing technical acquaintance about engine behaviour. Some of our B767 fleet had non-FADEC CF6-80As and others had FADEC C2s. On the former it was necessary to set the individual throttles to achieve symmetrical N1s, whereas on the latter the N1s were always matched even if there was slight throttle stagger.

Experimenting, it was observed that, on FADEC engines, if a small throttle stagger was set at cruise thrust, the N1s initially responded but then reverted to both indicating the same thrust. This seemed to be the case regardless of whether the autothrottle was in use.

On experimenting in the descent, the same behaviour was noticed until the stagger reached a larger displacement (several inches), when the engines then responded each according to its own thrust lever setting.

My questions were: at what point did the FADEC computer stop matching the N1s when the thrust levers were staggered? Was this point determined by differential thrust lever angle or differential thrust output? When the FADEC computer was matching the N1s, which thrust lever was the datum?

Reply:

BOEING OFFERS THE FOLLOWING IN RESPONSE TO REF /A/:

1/ PLEASE ADVISE AT WHAT POINT THE FADEC COMPUTER STOP MATCHING
N1 VALUES AS THE THROTTLE DIFFERENTIAL ANGLE INCREASES.

RESPONSE - BOEING ADVISES THAT WITH THE AUTOTHROTTLE ARMED,
THE FADEC COMPUTER (EEC) RECEIVES N1 TRIM (THRUST
MATCHING) INFORMATION DEVELOPED BY THE THRUST
MANAGEMENT COMPUTER (TMC) VIA THE ARINC 429 TMC
TRIM BUS. AFTER RECEIVING N1 TRIM INFORMATION FROM
THE TMC, THE EEC IS ABLE TO MATCH TRUST BETWEEN
ENGINES WHEN N1 VALUES BETWEEN ENGINES DIFFER UP TO
5% N1. WHEN THE N1 VALUES BETWEEN ENGINES DIFFER BY
MORE THAN 5%, THE EEC WILL EMPLOY MEASURES TO MATCH
N1 VALUES BETWEEN ENGINES AS CLOSELY AS POSSIBLE.

NOTE: THE N1 TRIM FEATURE IS UNAVAILABLE WHEN THE
AUTOTHROTTLE IS NOT ARMED.


2/ PLEASE ADVISE IF THE N1 MATCHING FEATURE IS A FUNCTION OF THE
THROTTLE ANGLE OR THE VALUES OF N1.

RESPONSE - BOEING ADVISES THAT THE EEC/TMC N1 TRIM FEATURE
MATCHES THRUST BY ADJUSTING N1 VALUES.


3/ WHEN THE N1 VALUES ARE BEING MATCHED BY THE FADEC SYSTEM WHICH
THROTTLE (LEFT OR RIGHT) IS THE DATUM.

RESPONSE - BOEING ADVISES THAT WHEN THE N1 VALUES BETWEEN
ENGINES DIFFER BY 5% OR LESS, THE EEC WILL ATTEMPT
TO MATCH ENGINE N1 VALUES TO THE HIGHEST COMMANDED
THRUST LEVER POSITION.


Surprisingly this engine behaviour was not mentioned in any of our tech manuals.
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Old 6th Jun 2024, 17:12
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Didn't happen when we had a flight engineer sitting behind us, just saying.
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Old 6th Jun 2024, 18:22
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Originally Posted by Discorde
...........RESPONSE - BOEING ADVISES THAT WHEN THE N1 VALUES BETWEEN
ENGINES DIFFER BY 5% OR LESS, THE EEC WILL ATTEMPT
TO MATCH ENGINE N1 VALUES TO THE HIGHEST COMMANDED
THRUST LEVER POSITION.


Surprisingly this engine behaviour was not mentioned in any of our tech manuals.
Was it anything to do with syncing the fan speeds together to avoid the sound of mismatched fans, I wonder ?, or is that only a propellor thing ?
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Old 6th Jun 2024, 18:49
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Originally Posted by Uplinker
Was it anything to do with syncing the fan speeds together to avoid the sound of mismatched fans, I wonder ?, or is that only a propellor thing ?
Interesting point. On our (non-FADEC) RB211 B757s you could manually match the N1s in the climb and cruise even if a slight EPR mis-match resulted in order to 'synchronise the props' and make the cabin sound less intrusive.

Last edited by Discorde; 6th Jun 2024 at 21:43. Reason: minor edit
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Old 6th Jun 2024, 21:05
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Originally Posted by Uplinker
Was it anything to do with syncing the fan speeds together to avoid the sound of mismatched fans, I wonder ?, or is that only a propellor thing ?
The FADEC 767 and 747 had autothrottle 'trimmer' that could make small (emphasis on small) adjustments to the thrust lever set N1/EPR to account for any throttle stagger. For N1 engines (GE), that automatically aligned N1, since that was the power setting parameter. For Pratt and Rolls, it aligned EPR - it wouldn't match N1.
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Old 6th Jun 2024, 23:38
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Originally Posted by Uplinker
Was it anything to do with syncing the fan speeds together to avoid the sound of mismatched fans, I wonder ?, or is that only a propellor thing ?
Not just a propeller thing. MD-80 had N1 and N2 sync modes. I think that when the sync mode selector was Off it was actually in EPR sync but it's a long time since I worked MD-80 systems. I think FADEC systems hid these sync modes from the crew and I don't remember any manual sync mode selection on MD-11 or B-717.
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Old 7th Jun 2024, 12:37
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Sounds like a known fault where it drops out regularly, and crew had got used to simply putting the switch back up and carrying on. Thatís the problem with recurring faults that donít get fixed. Have to say I would have been tempted to continue with manual thrust before reading this report (although hopefully setting and checking it was correct). A similar issue can occur in very strong headwinds when it goes into hold mode early because of the high IAS vs GS, before itís had chance to set T/O thrust.
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Old 7th Jun 2024, 14:46
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Ah, the old days. Precomputed EPR value set on each EPR indicator, big hairy arm of Flight Engineer pushes throttles forward to line pointers up with bugs. Quick look at N1 and other indicators followed by call of "Power set". The only things that went wrong were incorrect weight or wrong RTOW chart in which case FEs boot appeared behind throttle levers.
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Old 7th Jun 2024, 14:49
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Originally Posted by Propellerhead
Sounds like a known fault where it drops out regularly, and crew had got used to simply putting the switch back up and carrying on. That’s the problem with recurring faults that don’t get fixed. Have to say I would have been tempted to continue with manual thrust before reading this report (although hopefully setting and checking it was correct). A similar issue can occur in very strong headwinds when it goes into hold mode early because of the high IAS vs GS, before it’s had chance to set T/O thrust.
Heard about a problem like that on an Airbus. A/T dropped out every takeoff, PF reset switch. Unfortunately switch on one occasion was A/P. Interesting at Vr apparently.

Last edited by bugged on the right; 7th Jun 2024 at 17:09.
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Old 7th Jun 2024, 15:00
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Originally Posted by bugged on the right
Ah, the old days. Precomputed EPR value set on each EPR indicator, big hairy arm of Flight Engineer pushes throttles forward to line pointers up with bugs. Quick look at N1 and other indicators followed by call of "Power set". The only things that went wrong were incorrect weight or wrong RTOW chart in which case FEs boot appeared behind throttle levers.
Ah the glory days
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Old 7th Jun 2024, 16:08
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Hm, there are those target N1 markers on the (sorry I can't momentarily remember the term) engine instruments. Did the pilots just optically compare actual N1 to the target markers and these were stuck for some reason at 82(?)%. I know it's still unacceptable, but Human Factor, you know.

Aborting the takeoff: why not. I assume they noticed the failure and manually handled the levers. So they were just around 40% N1 and with snailing GS when TOGA failed? Good point to abort at almost no speed and discuss if you want to fly without AT support. Lots of assumptions, I know.
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Old 7th Jun 2024, 16:13
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Originally Posted by tdracer
the -3/4/500 (Boeing calls that the 'classic') or the -1/200 (aka the 'Jurassic').
Thanks, now I know how to name them. Having experienced the 300's in their heyday, I always struggled to accept "classic".
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Old 9th Jun 2024, 11:40
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Thumbs up

Originally Posted by beamer
With the end of the runway coming up at an alarming rate and way past V1, the expression ‘firewall the bastard’ comes to mind.
I was witness in the jump seat of a Boeing 737-200 to something like that. Dark night take off from a 5600 ft length runway on a tiny Pacific island. No overuns with both ends of the runway ending in the sea. OAT was 30C and Take Off max EPR of 2.18 was used. V1 was 121, VR 125 and V2 130. The F/O was PF but with the captain controlling the thrust levers. During the early part of the takeoff roll I felt the rate of initial acceleration was less than expected for 2.18 EPR but a glance at the engine instruments showed nothing abnormal with both engines showing identical readings.

With about four runway lights to go before the end of the runway it suddenly became obvious we were never going to reach VR before the end of the runway. I prayed the captain would not abort otherwise we would have gone off the end into the sea. The captain had by then realised we were in dire straits and calling "Taking over" he firewalled both thrust levers to their stops and hauled back on the control column. We were airborne at V1 minus 10 knots.

After setting climb thrust of 1.93 EPR and accelerating, flap retract was made and eventually we reached 250 knots climb speed. Rate of climb was lower than expected. Our attention turned to the N1 readings. From reading the tables in the Operations Manual it became clear that the N1 was 10% less than expected for 1.93 EPR. By the time we reached about 40 DME in the climb our height was significantly lower than normal. The captain elected to return to land.

After engine shut down at base, engineers checked the Pt2 inlet tubes in both engines and discovered both inlet tubes blocked by insect and other debris which later proved to include phosphate dust from mines on the island. It also transpired that engine covers normally used to cover the front of the engines overnight had not been in place that night.

Blockage of the Pt2 tubes whether by ice or other foreign objects has effect on EPR readings by making them overread. N1 are not affected. In this incident when the captain set 2.18 EPR with the thrust levers at the start of the takeoff roll, the real thrust would have been significantly less - probably around 2.10 EPR. However instead of the expected N1 readings of 100% N1, the N1 would have been around 90%. The N1 gauges in the 737-200 are quite small and in the low instrument lighting at night it would be difficult to differentiate between 100% and 90%N1.

Some months earlier I had experienced a similar problem during take off. During the takeoff roll around 80 knots the F/O announced "You are over-boosting" and tried to drag back one of the thrust levers under my hand. I saw a split in the EPR readings and directed the F/O to set 100% N1 on both engines. Once in the climb we could see a split in the engine readings more clearly. I suspected a Pt2 tube blockage on one engine as I had seen this sort of event before. I turned on the engine anti-ice switches (OAT 25C) and the problem righted itself even though it was obvious there had been no icing. From these experiences I paid closer attention to N1 rather than EPR readings during takeoff. N1 don't lie.

The AAIB report on the opening thread states:
Quote
"It is well known that humans are poor at detecting acceleration rates and recognising that their takeoff run is not matching the calculated performance. Performance issues can be insidious and invisible to the crew until very late in the takeoff roll. A previous report from the AAIB covered the reasons for this in detail1 . However, it is very unlikely that any crew will recognise there is an issue until they approach the end of the runway, and few crews then select an increase in power to try and mitigate their performance issues." Unquote.

I would go along with that..


Last edited by Centaurus; 9th Jun 2024 at 12:04.
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Old 9th Jun 2024, 22:45
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Originally Posted by Centaurus
I was witness in the jump seat of a Boeing 737-200 to something like that. Dark night take off from a 5600 ft length runway on a tiny Pacific island. No overuns with both ends of the runway ending in the sea. OAT was 30C and Take Off max EPR of 2.18 was used. V1 was 121, VR 125 and V2 130. The F/O was PF but with the captain controlling the thrust levers. During the early part of the takeoff roll I felt the rate of initial acceleration was less than expected for 2.18 EPR but a glance at the engine instruments showed nothing abnormal with both engines showing identical readings.

With about four runway lights to go before the end of the runway it suddenly became obvious we were never going to reach VR before the end of the runway. I prayed the captain would not abort otherwise we would have gone off the end into the sea. The captain had by then realised we were in dire straits and calling "Taking over" he firewalled both thrust levers to their stops and hauled back on the control column. We were airborne at V1 minus 10 knots.

After setting climb thrust of 1.93 EPR and accelerating, flap retract was made and eventually we reached 250 knots climb speed. Rate of climb was lower than expected. Our attention turned to the N1 readings. From reading the tables in the Operations Manual it became clear that the N1 was 10% less than expected for 1.93 EPR. By the time we reached about 40 DME in the climb our height was significantly lower than normal. The captain elected to return to land.

After engine shut down at base, engineers checked the Pt2 inlet tubes in both engines and discovered both inlet tubes blocked by insect and other debris which later proved to include phosphate dust from mines on the island. It also transpired that engine covers normally used to cover the front of the engines overnight had not been in place that night.

Blockage of the Pt2 tubes whether by ice or other foreign objects has effect on EPR readings by making them overread. N1 are not affected. In this incident when the captain set 2.18 EPR with the thrust levers at the start of the takeoff roll, the real thrust would have been significantly less - probably around 2.10 EPR. However instead of the expected N1 readings of 100% N1, the N1 would have been around 90%. The N1 gauges in the 737-200 are quite small and in the low instrument lighting at night it would be difficult to differentiate between 100% and 90%N1.

Some months earlier I had experienced a similar problem during take off. During the takeoff roll around 80 knots the F/O announced "You are over-boosting" and tried to drag back one of the thrust levers under my hand. I saw a split in the EPR readings and directed the F/O to set 100% N1 on both engines. Once in the climb we could see a split in the engine readings more clearly. I suspected a Pt2 tube blockage on one engine as I had seen this sort of event before. I turned on the engine anti-ice switches (OAT 25C) and the problem righted itself even though it was obvious there had been no icing. From these experiences I paid closer attention to N1 rather than EPR readings during takeoff. N1 don't lie.

The AAIB report on the opening thread states:
Quote
"It is well known that humans are poor at detecting acceleration rates and recognising that their takeoff run is not matching the calculated performance. Performance issues can be insidious and invisible to the crew until very late in the takeoff roll. A previous report from the AAIB covered the reasons for this in detail1 . However, it is very unlikely that any crew will recognise there is an issue until they approach the end of the runway, and few crews then select an increase in power to try and mitigate their performance issues." Unquote.

I would go along with that..
For said engine type, we had on our takeoff data card not only EPR but also min required N1. One of the calls on the takeoff roll was once the takeoff thrust was set. That call was "Takeoff thrust set, N1's check" (meaning that at least min N1's were being achieved. That solves the hazard of being unaware of not enough thrust being set from an engine instruments point of view.
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Old 10th Jun 2024, 04:56
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787 will match EPRs on the Trent 1000 even if autothrottles are not armed. You just need to nudge one of the thrust levers and the FADEC will tweak things as necessary. Makes it very easy to fly.
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Old 12th Jun 2024, 11:15
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Very late that I finally took the time to read the report.

Failure of the servo motor does not justify that an A/T system is called unreliable. motors fail, that's not a quirk.

To me it's now clear that the failure immediately happened on TOGA buttons application. So at very low speed. The crew would have the time to abort and discuss, or during low speed takeoff run to discuss an abort. With the reason to avoid flying hours without A/T avail. They didn't, which is tolerable of course.

When the A/T Arm switch disengages as a consequence of the failure, the N1 bugs with target values will disappear, right? And re-setting AT switch without selecting TOGA again leave the bugs disappeared? Is that possible to reset Arm and then reset TOGA at all, and does it depend on actual speed?

Another last Q: How about the lower sound frequency at almost 10 %-points less revs - pilots experienced on type should have a chance to detect it and take an extra glance on the Engine Instruments. Opinions?
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Old 12th Jun 2024, 11:43
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I used to know the rough fuel flows on take off and check if they were in that range..many moons later when I was writing junk, I asked a SFO at Dubai who had forgotten his crew ID and hence had to board his 777 with the pax going back to london as to what sort of fuel flows do you have on take off? He looked at me as though I was a moron and said he didnít know. Perhaps he was right.
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Old 12th Jun 2024, 13:16
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Originally Posted by waito
When the A/T Arm switch disengages as a consequence of the failure, the N1 bugs with target values will disappear, right? And re-setting AT switch without selecting TOGA again leave the bugs disappeared?
The bugs stay
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