737NG Reverser unlocked on Takeoff
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737NG Reverser unlocked on Takeoff
This came up as an interesting discussion point, and my cheapo FCOM2 doesn't have anything to say about it at all.
Should a reverser unlock on takeoff, the QRH says the EEC limits thrust on any engine to idle. This means you will get the thrust rolling back to idle. What it doesn't say is whether the thrust lever will move or not. I am guessing during the normal takeoff profile with THR HLD annunciated nothing will happen, but in ARM/MCP SPD does anyone know whether the thrust lever will move along with the command to idle thrust on that engine?
Reading between the lines on the QRH it seems you should be able to move the thrust lever, but the engine will not respond to demand, but I'd be interested to know what actually happens at various phases of flight.
Should a reverser unlock on takeoff, the QRH says the EEC limits thrust on any engine to idle. This means you will get the thrust rolling back to idle. What it doesn't say is whether the thrust lever will move or not. I am guessing during the normal takeoff profile with THR HLD annunciated nothing will happen, but in ARM/MCP SPD does anyone know whether the thrust lever will move along with the command to idle thrust on that engine?
Reading between the lines on the QRH it seems you should be able to move the thrust lever, but the engine will not respond to demand, but I'd be interested to know what actually happens at various phases of flight.
There is a FAR (and corresponding CS) 25.933 that basically requires that the thrust be limited to idle on an engine where the reverser isn't in the commanded position.
In the pre-FADEC days, this was typically done with a complex mechanical linkage that connected the reverser to the throttle cable system - if the reverser moved from its commanded position, it would move the throttle cable towards idle - in turn moving the throttle.
On Boeing FADEC aircraft (including the 737NG), it's all done with electrons. The FADEC monitors the reverser position and if it's not where it belongs (based on throttle position), it'll retard fuel and thrust to idle. The throttle itself won't move when in the forward quadrant. In reverse operation it works differently - there is a reverse 'interlock' which will prevent physical movement of the reverse thrust lever past idle until the reverser is deployed. In the unlikely event that the reverser then stows uncommanded, the interlock will return and 'snatch' the reverse lever back to idle.
BTW, if the reverser 'unlocks', but stays stowed, thrust won't be affected. I don't know the NG numbers, but we typically do is start cutting back thrust at 10% deployed, ramped to idle at 15% deployed.
In the pre-FADEC days, this was typically done with a complex mechanical linkage that connected the reverser to the throttle cable system - if the reverser moved from its commanded position, it would move the throttle cable towards idle - in turn moving the throttle.
On Boeing FADEC aircraft (including the 737NG), it's all done with electrons. The FADEC monitors the reverser position and if it's not where it belongs (based on throttle position), it'll retard fuel and thrust to idle. The throttle itself won't move when in the forward quadrant. In reverse operation it works differently - there is a reverse 'interlock' which will prevent physical movement of the reverse thrust lever past idle until the reverser is deployed. In the unlikely event that the reverser then stows uncommanded, the interlock will return and 'snatch' the reverse lever back to idle.
BTW, if the reverser 'unlocks', but stays stowed, thrust won't be affected. I don't know the NG numbers, but we typically do is start cutting back thrust at 10% deployed, ramped to idle at 15% deployed.
BTW, if the reverser 'unlocks', but stays stowed, thrust won't be affected.
Wouldn't that put you into only a 2 level of redundancy for the remainder of the flight with a flickering "unlock" light?
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Never had it in the real aircraft of course, however simulator behaviour implies that tdracer knows what he is talking about. With an unlock indication but stowed thrust reversers, thrust remains at its commanded level.
are you sure about that ?
Reasonably sure... I'm not that familiar with the specifics of the NG - never really worked on it (I've spent most of my career working the Boeing wide-bodies). But our FADEC T/R thrust cutback logic is pretty much the same across all models, and we've had occasional issues with nuisance 'unlock' indications so cutting thrust just because the unlock light flickers could cause other problems. I'm also reasonably sure that on the 737 the "unlock" indication is a separate system from the FADEC thrust cutback position monitoring system.
If the light comes on during takeoff, the guidance on how to proceed would come from the FCOM/AFM.
In any case, it's not the EEC that moves the throttles, it's the autothrottle. If the EEC is limiting the thrust, it will have no effect on the TL position, and the autothrottle will continue to try to do it's thing until it clicks off, similar to an engine failure.
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I can confirm Tdracer, had it in the sim, thrust lever doesn't move, looks like and engine rundown, except for the master caution (reverse light on the overhead panel) and the absence of the amber "fail" on the engine dial.
Can we clarify the sim condition?
Was it takeoff roll or after wheels up?
Does the sim distinguish between an unlock and/or reverser movement?
I've seen much higher failure rates on unlock detectors vs actual movement of a reverser. The challenge is to be prepared for the latter before it deploys at critical conditions faster than a pilot can reliably adjust..
Was it takeoff roll or after wheels up?
Does the sim distinguish between an unlock and/or reverser movement?
I've seen much higher failure rates on unlock detectors vs actual movement of a reverser. The challenge is to be prepared for the latter before it deploys at critical conditions faster than a pilot can reliably adjust..
