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Old 28th Nov 2010, 23:25
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Originally Posted by MachTwelve
...the FAA approved MMEL allows flight with the Beta Backup system deactivated, but states that reverse can not be selected. However when reverse is selected by twisiting the power lever grips, it de-activates the beta backup system anyway.
Hello MachTwelve:

There is a difference between "depowering" the beta backup system in accordance with the MMEL you refer to, and the "disarming" of the beta backup system that takes place during normal operations when you twist the power levers.

The nuance in the wording is this: If there is some form of defect with the beta backup system, the MMEL allows you to dispatch (for a limited number of days) with the system turned off. You accomplish this 'turning off' by pulling the circuit breaker for the system, and once this is done, the system does not function at all. The MMEL refers to this process as 'deactivation', in this context, they mean 'depowering'.

When the beta backup system is operating normally, once you twist the power levers and pull them aft of the idle stop (thus actuating the microswitch above the power levers that detects the grips being twisted), the beta backup system DISARMS for each propeller (individually) the moment that each propeller blade move to a position that is finer than +9° blade angle. Once the system has disarmed for one or both blades, it will not re-arm until the propeller blade moves out of the ground fine range, to a blade angle of +11° or more.

The purpose of the DISARMING (note the difference in wording between 'disarming' and 'deactivation') is to ensure that if the pilot rapidly moves the power levers forward - thus changing the state of the grip switch to indicate that the pilot has no longer selected ground fine range - the beta backup system will not begin to function as the propeller blade moves out of the ground fine range, back towards the idle (or coarser) blade angle position.

Consider this scenario: The pilot lands, then hauls the power levers all the way back to passenger seating row 3. The engines and propellers respond, and the propellers move to the full reverse (-15° blade angle) position. The pilot then rapidly shoves the power levers forward to the idle position, thus releasing the grip switch that indicates to the aircraft that the pilot has commanded ground fine range. The propellers will not move back to +11° as quickly as the pilot can move the power levers forward... there will be a delay of one or two seconds before the propellers move to the idle (+11°) position. If there was no disarming relay, the beta backup system would activate and cycle as the propeller(s) moved forward, because the grip switch would indicate that the grips were not twisted, but the propeller microswitch(s) would still indicate that the propellers were at a blade angle less than +9°.

Hopefully that explains the distinction between 'deactivated' (i.e. circuit breaker pulled) and 'disarmed' (a very specific condition governed by Boolean logic that looks at the sequence of activation of the grip switch and the propeller microswitch(s).

Originally Posted by MachTwelve
...Why can you not select auto-feather prior to descent?
Such an action is inappropriate for the DHC-6 aircraft, which has a different design of propeller control than the Beechcraft that you refer to. There are far too many engineering and certification criteria involved to explain them all here in the discussion forum, but, suffice to say that the subject has been most thoroughly investigated from an engineering point of view (I have read literally hundreds of pages of engineering analysis done when the aircraft was first certified back in the late 1960s), and it is inappropriate, undesirable, and arguably quite unsafe to select autofeather to 'ON' for landing the Twin Otter. The DHC-6 autofeather system is designed and certified for use during takeoff only.

Michael
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