MachTwelve:
You ask some very reasonable questions. I'm going to try my best to answer them, however, because it is no secret here in the forum where I work or what I do at work, I am going to have to be somewhat circumspect with my reply.
With the system depowered (via the MEL), why can you not select reverse?
I have no answer for that. That particular restriction has been present in the MMEL since before the Old King died. If I was to rewrite the MMEL today, I would not include that restriction. The beta backup system is only depowered (i.e. circuit breaker pulled) when there is a problem with the (electrical) beta backup system... under these conditions the (mechanical) propeller control system is operating normally. Hence, I cannot explain why that requirement to not select reverse is there.
It is
possible that the restriction to not select reverse arises as a result of a certification requirement that states that an aircraft equipped with a propeller that can operate in a ground fine range must be equipped with a system to annunciate when the propeller is, in fact, in the ground fine range. Because this annunciation system (the beta lights you see during normal ground fine operations) is powered from the same CB that you pull to depower the beta backup system, you lose the annunciation of ground fine range when you pull that CB. Hence, because there is no more annunciation of (normal, commanded) ground fine operation if you have pulled the CB, the MMEL then states that you must not invoke an operational mode (ground fine)that you know will not be annunciated to you. That is, however, a guess on my part.
I assume that the Beta Backup system was installed originally to avoid, the risk of inflight prop reversal at high torque settings...
No, not at all. If you think about it for a moment, the only possible way that the propeller can enter the ground fine range (in other words, zero thrust or reverse) is if the propeller is in an underspeed condition relative to what the pilot has selected with the prop levers. It is flat-out impossible for the propeller to enter the ground fine range if the power levers are significantly forward of the idle position (in other words, creating the 'high torque setting' that you refer to).
This is because if you have high torque present when the power levers are in the forward range, the propeller will rotate rapidly, and the propeller governor will reduce oil flow to the propeller hub in order to coarsen blade angle (for the purpose of limiting RPM to what you have selected with the propeller levers).
The above is precisely why the abnormal checklist for a cycling beta light (indicating mechanical failure somewhere, with the beta backup system operating correctly, as it was designed to do) tells you to move the propeller levers to the minimum RPM position - this to make it as easy as possible for the propeller governor to take control - then add a bit of power - that to increase propeller speed, so that the propeller governor becomes active, begins to govern, and restricts oil flow to the hub.
Why... has the FAA approved an MEL for it?
The beta backup system is analogous to a smoke detector in a house. If the smoke detector activates as a result of the presence of smoke, hey, it's doing what it was designed to do, and the 'memory items' on the checklist begin with "Get out of the house..." However, if the smoke detector activates and starts making a great noise and commotion, and subsequent careful investigation shows that there is no smoke anywhere (perhaps you banged it and caused something inside the smoke detector to break), then, the checklist for your house suggests that you depower the smoke alarm and then carry on as normal, taking note that you no longer have a warning system to advise you if smoke is present.
The MMEL entry for the beta backup system addresses circumstances when the backup system operates when it should
not operate (usually as a result of breakage of a microswitch out at the propeller). For example, consider that you are flying along in cruise, when suddenly one propeller slowly begins to feather. Torque rises at first (then drops as the propeller blade stalls), and Np drops dramatically from your usual 75% cruise to about 20 or 25%.
You look out the window, and see a propeller that is obviously feathered. Your leg muscles are throbbing from stuffing full rudder in to counteract the complete loss of thrust from the engine with the feathered propeller. But... one little beta light is illuminated, indicating to you that the propeller is in the ground fine range (less than +9° blade angle). You look out the window again and confirm that the propeller is at the full feathered position (+87° blade angle). The warning system - the beta backup system - is obviously crying Wolf, same as your smoke detector did. So, the checklist for the steady beta light tells you to do exactly what you did with your smoke detector - remove the power from the warning system that is raising the false alarm, then, go back to business as usual.
The MMEL entry allows you to dispatch from the next place that you land, and get the plane back to a maintenance base to get the "warning system" fixed.
Does the AFM contain an abnormal to deal with exactly this situation?
Yes. There are two abnormal procedures that deal with the beta backup system. The abnormal procedure for a STEADY beta light explains how to deal with a false alarm such as what I just described above. The abnormal procedure for a FLASHING beta light tells you how to deal with a mechanical failure of the propeller linkage that has caused the propeller to enter the ground fine range without the pilot commanding it to do so.
Note that neither of these procedures are memory items, and neither one requires fast or immediate action.
If you have a FLASHING beta light and you take a long time to complete the checklist, the worst that will happen is that the aircraft will yaw back and forth a little bit every time you reduce the power to a very low level... your passengers will think that you have a new co-pilot who is flying the plane while wearing 10 pound combat boots.
If you have a STEADY beta light and you take a long time to complete the checklist, the worst that will happen is that you will get a leg cramp from pressing on the rudder pedal (assuming that you don't apply trim). What you will have is, effectively, a single engine aircraft resulting from one propeller feathering itself in flight.
...Transport-Canada has approved the Kenn Borek STC to remove it
That is correct. Clearly TC thought that the STC was worthy of approval.
...and the 400 series doesn't even have it...
That is also correct. Clearly the manufacturer of the Series 400 Twin Otter thought that the system was not necessary, and TC agreed. Note, however, that the Series 400 does have a system to indicate to the pilot when the propeller enters the ground fine range. If the propeller enters the ground fine range when the aircraft is on ground
AND the pilot has commanded ground fine range (evidenced by twisting the power levers, which activates the switch above the power levers), the background behind the Np indication changes to a blue colour, as shown in the illustration below.
But, if the propeller enters the ground fine range when the aircraft is in flight, a red PROPELLER LOW PITCH Crew Alerting System (CAS) message appears, an aural warning "Propeller Low Pitch" is voiced, and the Master Caution annunciator illuminates. So, although the backup system itself has been deleted from the Series 400, annunciation of propeller entry to the ground fine range is retained - both for normal and abnormal circumstances.
Hope this answers your questions.
Michael
Series 400 Annunciation of Ground Fine Range (commanded)
