Bell 206 Gen Switch

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bj225
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Bell 206 Gen Switch

Postby bj225 » Mon Jul 23 2018, 09:22

Ive been told by a few people that if you turn the Gen Switch on when above 70 % N1, (say during flight ) that it could sheer the starter Gen where its connected to the Acc gearbox. Theres no mention in the flight manuals of either jetty or long of this limitation. Yet another person i spoke to said its a myth and can only sheer if the Starter/gen gets seized. pop;
Niko
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Re: Bell 206 Gen Switch

Postby Niko » Mon Jul 23 2018, 10:58

It's nonsense

Otherwise no-one would be able to perform Trend/Inflight Turbine Power Checks.
Check items of which include switching off the Generator and Anti-Icing/Bleed Air [in flight] and then switching Gen back on (and anti-icing if applicable) after the check has been completed
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Eric Hunt
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Re: Bell 206 Gen Switch

Postby Eric Hunt » Tue Jul 24 2018, 02:31

Never a problem on the B206, but on the A109A Mk2, it could be a problem if a gen was turned on before the second engine was started - both had to be done from the battery, or else quill shaft damage was probable by loading up a generator by trying to start the other engine from it. Too much load otherwise.

In a 206, particularly in flight, most of the load on the gen from recharging the battery after a start has finished, so turning it off and on in flight isn't a worry.
SuperF
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Re: Bell 206 Gen Switch

Postby SuperF » Wed Jul 25 2018, 12:31

interesting, I've been told the same and reading the manual wondered why it says turn off/on in flight.... Maybe its a from the old A models, or just an old pilots tale?
Geoff Williams
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Re: Bell 206 Gen Switch

Postby Geoff Williams » Thu Jul 26 2018, 00:45

Eric is correct. Here's where I think all this has come from, and has nought to do with the B206. It drifts a bit off topic to explain the whys so I apologise in advance.

This limitation applies to twin engine installations of the 250- series engines like in the A109, BO105 and AS355. The spline quill has a reduced radius, or neck if you like, to protect the accessory gearbox drive chain from damage in the event of a generator seizing or excessively loading the drive with the engine running. A weak link if you like.

The secondary effect of having this necking is when doing cross generator starts on a twin engine installation. The typical inrush current at initial pressing of the start button can be very high. Although of a short duration it can be as high as 800 plus amps. If the first started engine's generator is running when this happens, the shock loading on this gen can shear its drive shaft at the necking. At this point you think you have a gen failure on the first started engine until you shut it down and try and start it next time. Any inrush current or generator loading when turning a running generator on as a generator to a battery is only as great as the rating of the generator ( short circuits excluded) which is typically 150 amps, and well within the design limits of the quill shaft.

Solution, don't do cross generator starts on these types of installations. You can charge the battery if you need to after the first engine start during the time you are accelerating the engine into its governing range before starting the second engine so as to prevent 'slam engagement' of the second engine. Then switch off the gen prior to starting the second engine. For 'operational' quick starts I used a ground APU and started the second engine straight after releasing the starter of the first. This also ensured a smooth twin engagement as the first engine was still accelerating as the second one met it. Obviously monitor engine limits as per normal.

Interestingly Aerospatiale as it was then, in the early RFM for the 355F1 had both generator switches on in the pre start check list. Needless to say it was quickly changed after several failures.

Cheers
Geoff
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rotors99
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Re: Bell 206 Gen Switch

Postby rotors99 » Thu Jul 26 2018, 05:55

If all else fails..........follow the latest updated RFM :D & that sure as sheet says You can switch off the geni in flight :shock:

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