King Air 350 Environmental Bleed Switch
 

I have been sic on a 350 for a few years. Our s.o.p. is to move the environmental bleed switch to normal after gear retraction. We fly out of Los Angeles with frequent trips to Las Vegas. Needless to say heat is always an issue (106 the other day in Vegas). For you experienced King Air people would it not be better to leave the environmental bleed switch at low until perhaps 10,000' to facilitate better cabin cooling or should it be set to normal after takeoff?

I thought the flight manual recommended it stay in Auto for all operations to give the system control over whether it needs more cooling or more air? (Switch not fitted on the 200's I fly)

Why would low help cabin cooling?
I think you don't understand what that switch is doing.

Kalavo, Tourist: Please note I'm talking about the Environmental Bleed Air Switch, not the Cabin Temp Mode Switch. The Environmental Bleed Switch controls the amount of bleed air volume entering the cabin. In the Low position the amount of bleed air entering the cabin is reduced significantly. Since compressing the air in the cabin will be reduced it will also result in a slightly lower cabin temperature. According to the FlightSafety manual "LOW must be selected for takeoff when the ambient air temperature is above 10 Degrees Celsius. NORMAL may be used for increased heating or if increased pressurization air flow is required.

Tourist: If you have a better understanding of what the switch does please enlighten me. The switch will increase bleed air entering the cabin resulting in increased heat. You see anytime you compress air the temperature will rise (all other factors being equal) (In very hot weather the P3 Heat Exchangers are not very effective).

GregOates

From the Mx 300/350 Differences course.

Start of Quote

Low/Normal Switch Operation

There seems to be some confusion as to the purpose and operation of the Environmental Bleed Air LOW/NORMAL switch.

The switch is installed on the 300 series Super King Airs to meet takeoff performance parameters. For takeoff at ambient temperatures above 10 Deg C the switch must be in the LOW position.

In the LOW position, engine P3 extraction for airframe services is reduced 50% via the electronic flow control valves to allow the engine to meet takeoff parameters. In the NORMAL position, P3 extraction is back to 100% via the electronic control valves.

The Pilots' Operating Handbook states the operation of this switch for flight operations.

Reports of insufficient cabin heat and/or lack of pressurization schedule could be due to the switch being in the low position and an excessive cabin leak rate and/or malfunctioning control valves.

Therefore, these reports should be verified with the switch in the NORMAL position prior to doing any additional troubleshooting.

End of Manual quote.

To answer you question about it being better to use NORMAL earlier for high temperatures the resulting action for doing so would be to increase the amount of hot air into the cabin through the controls valves.

The only cooling you get without running the freon air conditioner is from the ambient air being mixed with the hot P3 bleed air. The P3 air is used to draw in the air from outside - Jet pump operation - and the resulting exit temperature is a result of the amount of P3 and ambient air mixed.

Yes the P3 air is cooled using the heat exchanger - but there are limitations as to how much it can be cooled. The P3 will always have a temperature above that of ambient and the outlet temperature amount depends on the ratio of P3 to Ambient as controlled by the electronic control valve and the actual ambient temperature and actual P3 temperature after it is cooled by the heat exchanger.

On a hot day the freon air conditioner should be used. The freon system is a recirculating system when pressurized and is the only real cooling that you will get on this aircraft as there are no air-cycle machinery present like the B1900.

On hot days the airplane will always be hot as the system is marginal at best. Turning the switch to NORMAL early is just going to add more heat into the cabin - something you don't really need.

Don't forget the "GOTCCHA" with low pressurization issues with the switch left in the LOW position for the rest of the flight.

B200 drivers can't help you on this one as this switch is not present on the standard B200 aircraft.

If you have arrived in the left seat without having a 300/350 course then you have arrived into a relatively high performance aircraft without training. Unless you have a super guy in the right seat - who should have been able to tell you all this - then you don't know the airplane and I suggest you find a course somewhere and take it. If the guy in the right seat hasn't had the course then you might find there are a few more things in this aircraft that can bite you.

There is no substitute for training - and contrary to what some people believe "a Beech is not a Beech is not a Beech" when it comes to the differences between models.

Hope this helps a bit.

He specifically stated he's in the right seat. Not that it's much of an excuse.

The best bet for rapid cabin cooling is leaving the bleed air switches off until line up. With the ECS on man cool or auto. As mentioned the bleed switches just introduce hot air into the system. A good right to left scan when doing the hot five will switch them on.

Vice versa on a cold day. Get the bleeds on ASAP.

Leaving the bleed switch in low may have a small effect. But it's meant purely for performance. And as soon as the bleeds are on the hot air is running round anyway.

As an aside I've seen all sorts of "personal techniques". All terrifying in their ability to go wrong. The top two are leaving the bleeds off until passing cabin cruise altitude and operating on one bleed only. Horrendous!!

Not a problem in the 90!

Thanks so much for all the info. I have flown with quite a few different captains and each one has a different take on the switch position and when it should be set to normal after takeoff.

what does the AFM say