DHC8 Autofeather

[tg743]

I've been browsing through the DHC8-200/300 technical information provided by Smartcockpit to see if I could learn something new.

Reading about the autofeather system i came across this.
After sending an uptrim signal to the non affected engine, the non affected bleed air system is shut down.

During my DHC8 (100 and 300) TR I never heard about this.

Anybody who have some background info on this? I assume that the bleed air is shut down only as long as autofeater is armed, and that it becomes available again when we deselect autofeather (and by doing so also cancel the uprtrim singal) during the engine failure shutdown checklist.

Atleast when doing engine failures in the sim, bleed air is available when engine is secured and cheklist complete.

[AerocatS2A]

First I've heard of it. If the bleed air gets automatically shut off, why do we get a performance penalty for having the bleed on for take-off? Anyway, I can't find any mention of the bleed being being shut off in either the flight manual or the maintenance manual.

[LimaFoxTango]

Never heard of it either. We operate with bleeds off for takeoff/landing anyway.

Edit:..

Just came across this as I went through an old groundschool powerpoint slide:

Autofeather occurs when torque on one engine falls below 28%. The autofeather sequence is now initiated. Immediately upon receiving and AUTOFEATHER signal, the good engine uptrims 10% torque.

- Displayed by a green UPTRIM light located to the left of the ECU switch.
-Torque on the good engine increases 10%.
- There is a 3 second delay then the Autofeather Solenoid Opens and Auxilary Feathering Pump runs for 18 seconds.
-The good engine bleed shuts off.
-Autofeather systems disarms.

Certainly this was something I was never aware of and I guess I was asleep during this part of class. Must try it in the sim next time. As I said earlier, we operate with bleeds off for TO/LDG.

[tg743]

We also have the bleed off for take off, and since thats the only phase of flight autofeather is armed it shouldn't be a factor anyway.
But thanks for your answers, I just find it interresting to dig into stuff I don't know, but I might should have known

[Apia Pipe]

Having previously worked many years on the Dash 8s, I was equally interested in this post, as it didn't ring any bells either. Just read through my training notes, and sure enough, there it is. In an autofeather situation, and also an overtorque situation, Power Uptrim light on, and non affected engine uptrims 10%, non affected engine bleed air shuts off ( I assume to allow max power availability).
After 3 sec delay, unaffected engine autofeather inhibited, and ARM light goes off. Back-up uptrim signal to unaffected engine, affected engine prop feathered. Deselecting the autofeather switch will turn the unaffected engine bleed back on again. That is a very basic description, as there are several other bits and pieces going on at the same time.
It must have been an afternoon session when we went over that part, and I was nodding off! Still, it proves that you never stop learning in this business!

[AerocatS2A]

I'd like to get an authoritative source for this. No offense intended, but I don't really trust someone's ground school power point and Smart Cockpit. As I said, I had a look through the Flight Manual and Maintenance Manual and neither mention it. Maybe it is a 100 series feature.

We have the bleeds on for take-off and landing.

[Apia Pipe]

AerocatS2A

The reference I was using was from my engineering training notes, which were a direct copy of the Bombardier training notes. Further to that, after blowing the dust off more notes, I have found a copy from the wiring manuals that show that in an autofeather situation, relays 6121-K15 or K16 (depending on which engine fails) energise which shut off the opposite engine bleed air. The only additional note I have, is that Mod 8/1747 disabled the latching function of these relays to prevent a sustained uptrim signal being caused by a momentary loss of torque signal. ie a transient dip. I seem to remember that there were occasions when the torque on an engine would 'hunt' slightly, and this could cause an uncommanded autofeather, even though the engine was nowhere near the trigger point. Over the years we suffered a lot of autofeather problems which tended to be caused by electrical connector problems because of the very short length of the pins. It became the practice to check tighten all engine connections with connector pliers, as 1 click on the connector could make all the difference. Improved engine harnesses were introduced, and we bought the Delastek test set to carry out fault finding especially on the TSCUs, EECs and the harnesses. Best bit of kit we ever bought. We also wrapped the engine connectors in self amalgamating tape to stop moisture ingress. We also discovered that the TSCUs needed to be matched part numbers, even though P and W said that certain unmatched numbers were compatible with each other.

Hope this helps

Apia Pipe

[AerocatS2A]

Thanks for that.

[tg743]

Thanks for your replies.

I'll se if I can find some time to look through our AOM today also.

[LimaFoxTango]

Here's a question. Would the bleed shut off during the Autofeather test? I'm thinking it doesn't as I haven't noticed it. Would be nice to know though.

[Apia Pipe]

LimaFoxTango

The old notes are really getting a good dusting!

According to the training manual, on carrying out the Autofeather test, relays 6121-K15 or K16 are energised, in which case the bleeds would be turned off. Basically, if the Power Uptrim light is on, then the opposite bleed from the feathered engine will be off, as the light is fed off the same relay(K15/K16).

I'll be interested to see if this is correct in practise. I must admit that it's ten years since I last worked on the Dash 8, but the series 100 and 300 were good aircraft once you got used to them. We did have a lot of problems with the autofeather, but certainly careful maintenance of the engine harnesses made life easier. The Delastek test set was magnificent, as it found faults on the TSCUs that the overhaul agencies couldn't find, and was pretty good at finding harness problems.