Auto throttle oddities B737-300/400
Autothrottle response oddities, anomalies and curiosities. (Plus some rambling as I get my thoughts down.). Quite likely these aren't that odd, and have been encountered many times before.
I'm curious about the operation of the autothrottle and interaction with the thrust levers on the 737 classic, with a couple of scenarios in mind. The scenarios being actual events, ongoing. The aircraft are B737-300 and -400.
Scenario 1.
Autothrottle armed for departure.
TOGA selected at or above 40% N1.
Target 'reduced' N1 in the region of 86-88%.
The thrust levers begin to advance as normal, but at about 75/80% N1, one thrust lever momentarily physically halts or even retards a fraction, requiring manual intervention. There is associated yaw until the thrust lever catches up.
Setting thrust at an initial higher N1 before selecting TOGA, e.g. Upto 50% prior to selecting TOGA has little effect.
Same aircraft, with the Autothrottle not armed. There is normal, free thrust lever operation, and normal thrust response while setting reduced take off thrust manually. I can't say specifically whether the various parameters such as N1, N2, EGT, fuel flow are matched and linear with the autothrottle off while setting thrust, but there is no obvious yaw.
Scenario 2.
Autothrottle armed for departure.
Setting the N1 to 40% (ish), a lag in acceleration- It manifests as a delay of about 2 seconds before the acceleration. The acceleration rate being similar between both engines, but with a lag.
Engines stable at 40/45%, then selecting TOGA. Right engine lags behind left, for a few seconds, and up to 20% N1 difference. Seems an eternity. Same effect with the A/T off. Once above 55% N1, the thrust lever response is normal.
Setting thrust initially at higher N1, e.g. 50/55% reduces the effect, but does not completely overcome it.
Maintenance checks of the A/T function invariable come back with no fault, suggesting (as opposed to proving) that it's not a fault of the autothrottle system itself.
In the above, on a rare occasion, intervention has continued as the FMA goes to 'Arm'. Hence why I've considered it appropriate to turn the A/T off.
Of course, a further exacerbation of the situation may come when the other fella selects TOGA early. It still happens quite frequently, especially with some more than others, and not necessarily low time pilots.
What might be the source of these anomalies?
Fuel control? Blade issues? Fan or turbine?
Operation of the autothrottle because it is controlling both throttles/engines?
Last edited by Lancelot de boyles; 7th Mar 2017 at 08:15.
Reason: Clarity