MachTwelve (what is your first name, anyway?):
Originally Posted by MachTwelve
The AFM abnormal I was actually referring to was 3.2.3 Inflight Propeller Reversal.
The procedure you refer to for 'inflight propeller reversal' (item 3.2.3 at revision 43 status) is totally unrelated to the beta backup system procedures. I realize that the causal factor for such a problem would, in theory, be something that should be caught (and prevented) by the beta backup system, but don't mix apples and oranges by trying to read too deep into the probable cause of the potential problem that this procedure addresses.
What the AFM says is (in summary) this: If a propeller enters the reverse range during flight and the aircraft does not have propeller blade latches installed, shut down the engine. If the aircraft
does have propeller blade latches installed (these are normally only found on aircraft that are operated on floats for all or part of the year), then don't shut the engine down, just move the power lever to idle.
That procedure makes sense. For your information, I have reviewed engineering, certification, and flight test data that shows that
with engine power at idle, a fully reversed (-15° blade angle) propeller actually creates less drag than the same propeller at +11° blade angle. This is because there is less flat plate drag presented at -15° than there is at +11°. So, the 'inflight propeller reversal' - a pretty remote possibility with the propeller control system installed on the Twin Otter - is not something that you should stay up late at night worrying about.
Originally Posted by MachTwelve
...can you comment on the option of twisting the power levers to dis-arm the system...
DON'T EVEN THINK ABOUT DOING THAT!
If you refer to the "Safety of Flight Supplement" number 4, dated 1 February 1981, which is found at the beginning of the Supplementary Operating Data (PSM 1-63-1), you will see that de Havilland
explicitly forbids doing this.
For either one of the beta malfunctions (steady light or flashing light), take your time, go slowly, correctly identify the nature of the malfunction, correctly identify the right checklist to use and get agreement from the other pilot (if you are in a two-crew environment), then carry out the checklist actions as "read aloud and do", not from memory.
As I said earlier, if you were to freeze up in terror and do absolutely nothing in response to a steady beta light, the only consequence would be that you would get a sore leg from holding in all the rudder during the single-engine flight that would result. If you were to freeze up in terror and do absolutely nothing in response to a flashing beta light, the only consequence would be that when you finally brought the power back for landing, the aircraft would yaw back and forth a little bit (entirely controllably) during the flare.
However... if you act in haste and screw up and carry out the wrong procedure (e.g. complete the 'steady beta light' procedure when the problem is in fact a 'flashing beta light'), the consequences could be quite severe.
You have to keep in mind that the ONLY time that the propeller can go into a very fine pitch (meaning, less than +11°, with the reverse range from 0° to -15° being sort of an 'ultra fine pitch') is when the propeller is underspeeding relative to the propeller governor. This means that
during the takeoff, climb, and cruise phases of flight, it is impossible to get a flashing beta light, because during all those phases of flight, because the large amount of power (torque) being applied to the propeller is keeping it under the control of the governor, which modulates the oil supply in order to maintain the propeller RPM selected by the pilot.
The only time that the 'flashing beta light' malfunction can appear is during the approach and/or landing phase of flight, when the power being applied to the propeller is reduced to such a low value that it is insufficient to keep the propeller turning at the RPM selected by the pilot. If the flashing beta light malfunction does appear, and the pilot adds power (one of the two items the checklist calls for the pilot to do), the addition of power will increase propeller RPM and hand control of the propeller back to the governor. The problem will disappear, at least, it will disappear until the power lever is next pulled back and the propeller is unable to maintain the speed selected using the PROP lever.
Originally Posted by MachTwelve
...don't really understand why it was mandated and thence effectively (via both the STC and the 400) the requirement was removed.
It was mandated because the design of the propeller control system used on the Twin Otter - which provides direct control of propeller blade angle via the power lever during approach and landing - was considered new and novel when it was first introduced in the 1960s. Therefore, the regulators at the time asked for an independent electrical (rather than mechanical) backup system to prevent the propeller from entering the ground fine range during flight should the mechanical system somehow fail. This was prudent on their behalf and entirely understandable given the lack of flight experience with this new and novel propeller control system.
40 years later, we have the benefit of having learned from (literally) millions of hours of Twin Otter flight time that:
1) If the mechanical propeller control system fails, 99.9% of the time, the result is that the propeller feathers. That is the least hazardous thing that the propeller could do as a result of a failure, and no back-up system is needed if the propeller feathers.
2) Although the theory behind the beta backup system is sound, in practice, the system causes more problems (unwanted featherings as a result of the 'steady beta light' scenario) that it prevents. Therefore, regulators have approved STC modifications permitting removal of the backup system - although the INDICATION system (lights that show when the propeller is in the ground fine range) must remain.
Be aware, however, that during the 1980s, de Havilland introduced an optional modification to the design of the beta backup system (Mod 6/1831, S/B 6/478 at Rev A refers) that replaced the propeller mounted microswitches with a proximity switch, and this modification has virtually eliminated all 'steady beta light' malfunctions, except of course for those arising from a rigging problem, which would in any case be detected on the ground as soon as the engine was started up following the maintenance activity. This modification can be easily retrofitted to all legacy DHC-6 aircraft.
In the case of the Series 400, a very clear alert (text message and voice announcement) is made to the pilot if the propeller ever enters the ground fine range during flight. The actions that the Series 400 pilot then takes are identical to the actions set out for the "flashing beta light" for a Series 300 aircraft.
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You may also want to review and compare the actions set out in the AFMs of single engine aircraft that use a similar engine and propeller control system (e.g. Cessna Caravan, Pilatus PC-6, Pilatus PC-12) and do not utilize a 'high idle' fuel control position such as the Beech products. Aircraft that utilize a 'high idle' position are, mechanically, sufficiently different that it is not appropriate to make a direct comparison with the Twin Otter.
Michael