Caravan Pilots

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Author:  rhinoboy55 [ Fri Aug 06, 2010 10:12 pm ]
Post subject:  BYPASS USE

Over the years I have watched the use of the pilots engine air bypass and have these thoughts. It is in the manual that is supposed to be used for icing (on the spinner) below -4c or when there is heavy precipitation or on the ground when it is dusty but here is my argument. If you watch the plane on the ground when it starts the centrifugal force from the prop sends out all the incoming air out and around the cowling therfore making the cleanest air available directly through the front of the engine at the intake. And the dirtier air that has been sucked up by the prop goes around the side of the cowling where it is ingested into the engine. I don't think there is enough air speed to keep the bypass mode from effectively seperating the dirty air when it is on the ground in beta and even reverse sometimes. So I dont think I would use the bypass on the ground on an aircraft I owned. Others say you should use it on takeoff to keep birds out, but the larger screen inside the engine is supposed to do this. Any comments welcome.

Author:  Lazy Eagle [ Sat Aug 07, 2010 3:26 am ]
Post subject:  Re: BYPASS USE

Makes since what you are saying. If there is not good forward movement, the turbine is probably sucking from both ends. Anybody know about this?

Our plane has the larger tires and the extended fork. Seems darn high to suck something off the ground. Probably as safe as regular jets at this height.

Author:  werbil [ Mon Aug 09, 2010 12:11 pm ]
Post subject:  Re: BYPASS USE

On amphibs on dirt, in certain conditions you can see a vortex of dust drawn off the ground into the propeller blade tips. The spinner on an amphib is about 10' off the ground so I doubt the extended fork and big tyres will prevent dirt and rock ingestion.

Whilst the propeller control is full forward and the power lever is in front of the gate there is plenty of air for the air conditioner to operate correctly and for the oil cooler to work even when facing downwind so I expect there would be plenty of air for the separator to operate correctly. If not Cessna would not be recommending as they do.

However, if the propeller is feathered or in Beta or Reverse for any period that is a different story - the air conditioner cycles due high pressure if the switch is working or blows off hose fittings if not. I suspect the reason for the 60 second beta / reverse limit will be for oil cooling. Facing downwind in feather for extended periods you do have to monitor oil temps to ensure that they stay in the green. Whether the separator functions as advertised in these questions I don't know. Next time I'm on the floats with the engine in feather and separator open I'll take note of whether the outlet is sucking or blowing.

We only use reverse whilst still at high speed to avoid blowing up dirt / water. Creating avoidable dust in the area of the engine and then relying on the separator to get rid of it is stupid IMHO. I can't see centrifugal effects having any effect on dirt in the propeller disc as air is drawn longitudinally through the propeller rather than being thrown out - the vortex being drawn up from the ground into the blade tips of a feathered propeller suggests to me that the tips are in an area of low pressure.

Author:  Lazy Eagle [ Mon Aug 09, 2010 1:45 pm ]
Post subject:  Re: BYPASS USE

Werbil, good information. Let us know what you may find more of.

Couple of questions?

I did not know there was a 60 second rule during feather, is this in the book? We've been told longer is o.k., but monitor the gauges.

When in feather at low idle, debris may be pulled near the tips, but should not rise through the air intake. I don't have a clue at high idle? Salt mist would be horrible!

Author:  werbil [ Mon Aug 09, 2010 10:48 pm ]
Post subject:  Re: BYPASS USE

Lazy Eagle

I'm pretty sure that the limit for reverse and beta is in the engine limitations table in Section 2. There is no time limit that I am aware of for operation in feather - on occasions we sit in feather for up to twenty minutes whilst we hot load to reduce our engine cycles.

I have seen the dust vortexs on vans in high idle (another operator based at our field used to taxi at high idle). The worst conditions for picking up salt spray with the prop is when taxying downwind in over about 15 knots (we have to do this to enter one marina and to get to the beginning of take off runs at the reef). Unfortunately there is nothing that we can do to prevent this spray pickup - if there is water going onto the screen the separator is open.

IMO Intentionally operating a serviceable aircraft on salt water is one of the hardest things you can do to an aircraft. Corrosion is an ongoing battle for our engineers - the caravan was never designed for this type of abuse. All I can say is I'm glad I don't own the aircraft.


Author:  Lazy Eagle [ Tue Aug 10, 2010 2:30 pm ]
Post subject:  Re: BYPASS USE

Thanks Rhinoboy55 and Werbil, your information is great. I copied and moved the discussion from Maintenance to the General Discussion Forum as there may be some pilots who will lead on.

Lazy Eagle...

Author:  werbil [ Mon Aug 23, 2010 9:25 am ]
Post subject:  Re: BYPASS USE

In feather the outlet definitely sucks air in - checked today. I spoke to a specialist engineer who was doing a HSI on one of our vans a few days ago and he explained that they are inertial and require a lot of airflow to work. I asked his thoughts about which was best on the ground at low idle but he was non committal.

More thought required on this one.

Author:  rhinoboy55 [ Sat Sep 04, 2010 4:26 am ]
Post subject:  Re: BYPASS USE

Werbil, Thats good you watched it. I think it is a bit of a paranoia to use it all the time with owners cause the engine is so expensive. I think I will NOT be using it on the ground. Or for takeoff unless it is raining like crazy, but I've taken off through torrential rains before without it(accidentally) and nothing happened, I mean it ran smoothly. Thanks again

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