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Guest Peter Bowcut

Question RE: 747 autothrottle operation (real-life)

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Guest Peter Bowcut

I was under the impression that the -400 autothrottle system completely managed throttle position from start to finish. In other words, when the TOGA buttons are pushed, I thought a servo motor automatically advanced throttles to takeoff power setting. In another recent thread here, someone said that the autothrottle system merely 'trims' the power setting and the pilot must still manually advance the throttles for takeoff thrust, even after pressing the TOGA switch. Is this correct?Also, I've read on other websites that the throttle position itself controls thrust setting according to the VNAV reference and that if the autothrottles were disengaged overboost protection was lost. I thought the the EEC system electronically controlled max power output regardless of whether or not the autothrottle system were engaged (this via a fuel metering control unit slaved to the EEC). In other words, I thought that if the autothrottle was disengaged/disarmed the EEC system in normal mode still provided overboost protection. Am I confused?Thanks!

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>I was under the impression that the -400 autothrottle system>completely managed throttle position from start to finish. >In other words, when the TOGA buttons are pushed, I thought a>servo motor automatically advanced throttles to takeoff power>setting. In another recent thread here, someone said that>the autothrottle system merely 'trims' the power setting and>the pilot must still manually advance the throttles for>takeoff thrust, even after pressing the TOGA switch. Is this>correct?>>Also, I've read on other websites that the throttle position>itself controls thrust setting according to the VNAV reference>and that if the autothrottles were disengaged overboost>protection was lost. I thought the the EEC system>electronically controlled max power output regardless of>whether or not the autothrottle system were engaged (this via>a fuel metering control unit slaved to the EEC). In other>words, I thought that if the autothrottle was>disengaged/disarmed the EEC system in normal mode still>provided overboost protection. Am I confused?>>Thanks!Hi Peter,No, this is not correct. When the TOGA buttons are presses, the auto-throttles take over and sets the correct N1 that's programmed in the FMC. It's true that the pilot will keep his hand on the throttles because this is the correct procedure in the unlikely event that somethings bad happens prior to V1. But he doesn't push the throttles. The throttles move themselves. When the first officer calls out V1, the pilot then removes his hands from the throttles because from now on, the pilots are committed to take-off.In your second question, I don't think that VNAV reference has anything to do with the postions of the throttles, but Steve or one of you real world 747 pilots, correct me if I'm wrong. I think you're correct that the EEC provides boost protection even though the auto-throttle system is disengaged, but again, Steve or someone correct me if I'm wrong.Ken.

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Ken, in the main you are correct.VNAV is one of several inputs used by the Autotrottle system.Overboost protection is controlled by the EEC's not the Auto throttle system. There is a lot of integration in the 747 autoflight system and there may be several different subsystems determining the commanded AT position.CheersSteve


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Steve Hall

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The comments re: handling of the thrust levers after engaging TOGA were mine. I should clarify: The thrust levers don't just 'trim' the power to the correct setting, if you leave the thrust levers alone they will advance to TOGA without assistance, however I've seen a few videos that imply the PF selects TOGA, then 'assists' the thrust levers through to about the correct setting before holding their hand there until V1.Have I correctly understood this or do the crew not apply any positive pressure to the thrust levers after selecting TOGA?


Mark Adeane - NZWN
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Guest Peter Bowcut

>The comments re: handling of the thrust levers after engaging>TOGA were mine. >>I should clarify: The thrust levers don't just 'trim' the>power to the correct setting, if you leave the thrust levers>alone they will advance to TOGA without assistance, however>I've seen a few videos that imply the PF selects TOGA, then>'assists' the thrust levers through to about the correct>setting before holding their hand there until V1.>>Have I correctly understood this or do the crew not apply any>positive pressure to the thrust levers after selecting TOGA?The autothrottles will indeed move by themselves automatically as 747 pilot Steve (Cowpatz) described when engaged, yes. The flightcrew manually 'backs up' the automatic servo motor for the autothrottle in case a friction clutch fails in the auto system (could create an assymetrical thrust situation if this happened.), or if the servo motor itself were to fail. The pilots aren't applying any force to the autothrottle, but it looks as if they are in some videos. The autothrottles will go into HOLD mode when fully set forward for takeoff, but I believe the reason the pilot keeps his hand on the throttle until V1 is in case of an abort he will be ready to chop power quickly. After V1, the aircraft is committed to flight and he removes his hand and places it on the yoke. I think his hand on the throttle until V1 is also a method of procedure, reminding him that he is still below maximum abort speed until reaching V1, and committed to takeoff after V1 (both hands on yoke).Thanks for all the replies, cool stuff you guys!

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The only reason to manually assist the throttles would be if one of them was lagging behind the others due to a slipping clutch. It may also be necessary to fine tune and even up the TO power settings. It's hard to tell from a video whether the pilot is pushing the levers or merely following up the movement.Kevin


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As has been mentioned the thrust levers are not pushed up but followed up. The thrust levers are "stood" at approx the 70% N1 position momentarily (to ensure all engines spool up and stabilise) and then the TOGA button is depressed. It could get quite exciting if this procedure were not followed and an eng (especially an outboard) failed to spool up. Arguably one of the worst engine failures to have is an outboard eng failure prior to 80 kts. Normally the Captain will remove his hand from the thrust levers just before V1. V1 is a GO speed, so having the hand off slightly early ensures the mindset is switched to GO.CheersSteve


Cheers

Steve Hall

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Cheers for the clarification guys. Was in the 737NG fixed-base sim tonight and asked one of our 737 drivers about this, he concurs that it is a backing up action and that you don't push the thrust levers. On putting ths into practice it makes sense.You learn stuff all the time around here!EDIT: I should also offer my apologies for putting people wrong in the original post when I said that you DO push them through, sorry guys! :-shy


Mark Adeane - NZWN
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"Also, I've read on other websites that the throttle position itself controls thrust setting according to the VNAV reference and that if the autothrottles were disengaged overboost protection was lost."The EEC's have their own set of limits (not unlike the FMC's) for protection when the A/T is not engaged. However, if you manually pushed your throttles forward to full takeoff power and left them there more than the recommended 5~10 minutes, you would eventually cook the engines (with excessive oil temps). Switching the EEC's to ALTN also runs the risk of damaging the engines. There are additional non-electronic limiters which help protect the engines (but you wouldn't want to rely on these as the limits are set quite high).As previously stated, it's normal to set the engines to a fixed EPR/N1 value, manually, prior to pushing the TOGA switches (e.g. 1.1 EPR or 70%N1), but the A/T system should respond to the TOGA button push with the engines at idle.Cheers.Q>

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Guest flyandive

I had the fun experience of getting trained on the MD-80 at Continental (30 hrs of sim time). At Cal they did assist the the thrust levers up because of a quirk the airplane had, so perhaps in some videos the pilots came from this mentality? The MD-80s autothrottle would set the takeoff thrust but the catch was, because you were supposed to have the thrust set at 60 kts due to field length requirements, that at 60kts they would "clamp." Meaning the autothrottles would "let go" and not do anything, to allow the pilot to abort without having to worry about the disconnect. All fine and dandy on a calm wind day but if you had a 20kt headwind then they would clamp 20kts too soon, before thrust was set. So at Cal they would advance the throttles to 1.4 EPR, let them stabilize, engage the autothrottles, then gently help them to ensure they set properly. My thought is maybe Cal isn't the only airline that taught this and so the mentality carried when these pilots transitioned to other aircraft.-Chris

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