Boeing 737 and FAR 25 aircraft thrust reverser unlock malfunctions
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Boeing 737 and FAR 25 aircraft thrust reverser unlock malfunctions
Hi All,
1. For the B737-300/500, why does it not mention on the reverser unlocked QRH that the affected thrust lever will snap back if you this occurs or even in the FCOM of the engines section?
See reference from approximately 40 secs onwards in the video
I have used this to simply illustrate the system effects of this only!
2. For the B757/B767/F100 etc… with non FADEC engines does the thrust lever snap back to idle on these similar vintage aircraft as well?
3. Why on the B737-600/900 does the pilot not have the same tactile feedback for the reverser unlocked as the B737-300/500, you would think it would provide a good tactile feedback for the pilots?
4. As I understand on the classics and the NG, when the unlock malfunction is encountered and corrected by the system there is still some form of idle reverse can someone confirm this?
1. For the B737-300/500, why does it not mention on the reverser unlocked QRH that the affected thrust lever will snap back if you this occurs or even in the FCOM of the engines section?
See reference from approximately 40 secs onwards in the video
I have used this to simply illustrate the system effects of this only!
2. For the B757/B767/F100 etc… with non FADEC engines does the thrust lever snap back to idle on these similar vintage aircraft as well?
3. Why on the B737-600/900 does the pilot not have the same tactile feedback for the reverser unlocked as the B737-300/500, you would think it would provide a good tactile feedback for the pilots?
4. As I understand on the classics and the NG, when the unlock malfunction is encountered and corrected by the system there is still some form of idle reverse can someone confirm this?
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For the B737-300/500, why does it not mention on the reverser unlocked QRH that the affected thrust lever will snap back if you this occurs or even in the FCOM of the engines section?
Under the heading Reverser Unlocked (Inflight) it states (in part) "Movement of the reverser sleeves to the reverse thrust position mechanically retards the thrust lever to the idle position, and the interlock limits movement of the thrust lever as long as the engine is in reverse thrust."
This statement does not appear in later issues of the FCOM or in the QRH. I don't know therefore if that statement is still valid but certainly in the 737 Classic simulators of that era which are still used, that statement is still true. Maybe an autostow modification is the answer to your question - but that is only a guess.
The following is common for all Boeing installations - AB and Douglas/MD implementations are not necessarily the same.
The throttle snap back and power reduction is based on the actual T/R cowl position. If it's only 'unlocked', nothing will happen - the reverser needs to be ~10-15% deployed before the thrust cutback (and non-FADEC thrust lever retard) takes effect. On non-FADEC engines this is done by a cam in the strut box that is positioned by cables connected to reverser sleeves - when the reverser moves far enough towards deploy, that cam restricts the thrust lever position to idle. That same cam restricts the throttle to reverse idle during 'normal' T/R deployment - then allows full reverser thrust lever movement once the reverser is ~90% deployed.
FADEC uses electrons to do the same thing, the throttle doesn't move for uncommanded reverser movement in forward thrust. In reverse, the FADEC engines have a mechanical interlock that prevents reverse lever movement beyond reverse idle until the reverser is ~90% deployed (to give a tactile indication of T/R position). There is no throttle snatcher in forward, but in reverse if the T/R stows uncommanded, the reverser interlock will snatch the reverse lever back to reverse idle. Uncommanded stow is very rare, but if it happens watch your hand - it can easily trap it between the levers and do serious damage.
Post Lauda - with the addition of the third lock - the current cert basis for Boeing thrust reversers is that they won't deploy in-flight. No first hand knowledge here, but perhaps the AFM/QRH reference isn't there since it supposedly will never happen. Note that this is applicable to 'high bypass' engines - I don't believe the third lock was ever mandated for the low bypass engines (e.g. JT8D on the 727 and 737-1/200).
The throttle snap back and power reduction is based on the actual T/R cowl position. If it's only 'unlocked', nothing will happen - the reverser needs to be ~10-15% deployed before the thrust cutback (and non-FADEC thrust lever retard) takes effect. On non-FADEC engines this is done by a cam in the strut box that is positioned by cables connected to reverser sleeves - when the reverser moves far enough towards deploy, that cam restricts the thrust lever position to idle. That same cam restricts the throttle to reverse idle during 'normal' T/R deployment - then allows full reverser thrust lever movement once the reverser is ~90% deployed.
FADEC uses electrons to do the same thing, the throttle doesn't move for uncommanded reverser movement in forward thrust. In reverse, the FADEC engines have a mechanical interlock that prevents reverse lever movement beyond reverse idle until the reverser is ~90% deployed (to give a tactile indication of T/R position). There is no throttle snatcher in forward, but in reverse if the T/R stows uncommanded, the reverser interlock will snatch the reverse lever back to reverse idle. Uncommanded stow is very rare, but if it happens watch your hand - it can easily trap it between the levers and do serious damage.
Post Lauda - with the addition of the third lock - the current cert basis for Boeing thrust reversers is that they won't deploy in-flight. No first hand knowledge here, but perhaps the AFM/QRH reference isn't there since it supposedly will never happen. Note that this is applicable to 'high bypass' engines - I don't believe the third lock was ever mandated for the low bypass engines (e.g. JT8D on the 727 and 737-1/200).
The Lear 45 which is also FAR 25 has thrust lever snap-back. It’s quite powerful in the sim. It was one of the rare events where you could shut an engine down without confirmation (and indeed had to be bloody quick to do so) as the failed side was already in idle and just needed to be lifted straight to cutoff.
The TREs at CAE also taught instinctive knowledge of the position of the deploy circuit breaker and pulling that immediately as well. If given at V1 it was the most challenging failure to handle on the aircraft.
The TREs at CAE also taught instinctive knowledge of the position of the deploy circuit breaker and pulling that immediately as well. If given at V1 it was the most challenging failure to handle on the aircraft.
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A quick question about the reverser unlocked. Sim scenario, taking off and during the acceleration after V1 you expect a “rev unlocked” then after a bit and eng failure. Which checklist will you pick first?
depends on the consequences, The specific manual shouldl cover it
