I guess there are two parts to your question. First is figuring out temperature aloft. The quick way to do this is to use the standard lapse rate of 2°C (3.5°F) per 1,000ft. For example, if the airport is at 1,000ft MSL and the temperature is 35°C and you want to find the temperature at 10,000ft, you should subtract 2°C for every 1,000ft of altitude change. That puts the temperature at 10,000ft MSL at about 17°C for those conditions. If you want a little more accurate temperature information you can get it from various aviation weather products such as the winds aloft:
http://aviationweather.gov/products/nws/winds/

The second part of your question is about when to use pitot heat. In reciprocating engine aircraft, pitot heat is typically turned on prior to flight into visible moisture. This is usually done even if the temperature is above freezing. If the temperature is around freezing but there is no visible moisture, there is no need for pitot heat. Most jet aircraft will leave the pitot heat on for the entire duration of the flight, regardless of the weather conditions.

C wrote on Sep 12^th, 2009 at 4:21am:


I think the Pitot Tube does consume a fair few Amps from the Power supply, which probably affects smaller GA Aircraft with limited Battery Power.

..and of course, those of us with piston prop, normally aspirated engines, also have to remember to apply our Carburetor Heat at regular intervals, (especially before closing the Throttle), the prevent Carburettor Icing! ....again this puts a strain on the Battery if left on unnecessarily.

Its always a good idea to remember to apply Pitot Heat, and Carburettor Heat at regular intervals during the flight, whilst keeping a careful watch on the instruments, and listen to the sound of the sound of the Engine(s) for possible malfunction...It will help in the "Real Flight"... ...!

Paul

Hagar wrote on Sep 12^th, 2009 at 4:51am:


...quite right, Doug....

...Knickers in a twist again... ..!

Carb Heat affects the Power available.

I've got so used to applying Carb Heat on all my little Cessna 150/152's, that its now become automatic to use it when necessary....

Apply heat before closing the Throttle prior to landing, and cancel Carb Heat before touch down if an aborted landing becomes necessary and maximum power is required for go-around, ....or a touch-and-go.. ...!

..the same Icing problem affected all my "Old" Motor Bikes, and Vintage Motor Car, fitted with Carburettors, in adverse Cold + High Humidity Weather conditions.... ....!

Paul...just heating up a nice Cup of Tetley Tea... ...!

Fozzer wrote on Sep 12^th, 2009 at 5:33am:



I wouldn't be doing anything else right before touchdown except flying the airplane!  The flare and landing should have your full attention.  You shouldn't be fumbling around with the carb heat.  It can be closed after touchdown.  Should a go-around be needed, carb heat in, full power!

Also Brett.... #2 - Never on until airborne.

Pitot Heat is usually a line-up checklist item.  I always turn it on while taxiing on to the active, along with transponder and lights.  If you have to wait awhile before takeoff, then leave it off.

Brett_Henderson wrote on Sep 12^th, 2009 at 11:56am:


That Carb' Heat technique has become second nature for me now, in my little Cessna's....so that I don't forget!.... ...!

Paul...FS 2004... ...!

carb heat.  = on BEFORE Idle, off BEFORE bat out of hell.  -- gotcha.

pitot heat = (ugh i hate metric scale being i live in Farenheight land) -- (reported ground temp) - 3.5º F / +1000' MSL) = Temp aloft ???




90º summer day at KCHS .. will be flying at FL200 to KMIA VFR.... KCHS @ +46 ft MSL,

so..... may as well get use to turning it on anyways? even though ground temp is 90º.      

just flip the switch when atc says "you are cleared for take off"

I should point out that quite a lot of light aircraft have no carb heat control or heated pitot head. I'm thinking specifically of the Tiger Moth but there are many others.

Quote:

Paul. I've noticed a worrying trend towards Americanisms in your recent posts. First carburetor & now this.

Capt.Propwash wrote on Sep 12^th, 2009 at 3:39pm:


Temperature has no bearing on pitot heat.  Most GA planes turn it on before entering visible moisture (clouds), regardless of temperature.  Turbine aircraft turn it on before takeoff and leave it on until landed.  Pretty simple.

Good thread.

Just for clarification, carb heat doesn't take power from the engine. There is a cowl surrounding the exhaust which air is allowed to blow through, absorbing heat. This air is diverted into the carburetor. The hot air will change the fuel mixture, causing a slight loss of power. It's like a sudden density altitude change for the mixture. Less oxygen to burn, so the mix gets richer.

flaminghotsauce wrote on Sep 14^th, 2009 at 5:43am:

Yes, but effectively, it takes power away... maybe 1-200 rpm, but it's very noticeable when you're already losing rpm because of ice in the carb inlet... those few seconds (if you're lucky) when the carb heat is on and the ice hasn't melted can be a little disconcerting.   

Many unexplainable Aircraft crashes can be put down to Carburettor icing, but the ice has already melted in the Carburettor by the time the investigation is carried out, so there is no firm conclusion... ...!

As a ground check before take off, apply full engine revs with the brakes applied, and select Carb Heat. The engine revs should drop by approximately 150 RPM to ensure the system is working correctly.

The dangers of Carburettor Icing during flight, frightens me!...(even in the Sim!)...so I regularly apply it!....!

Paul...G-BPLF...FS 2004... ...!

... ...!

DaveSims wrote on Sep 14^th, 2009 at 9:30am:

Just to clarify my clarification: I probably wasn't clear when I said it doesn't take power from the engine. When I say "take power from the engine" that's more like turning on an air conditioner that actually takes power... from the engine. Carb heat doesn't "take" power, but it prevents the power from being produced by changing the mixture.

Semantics I know.

I've pulled the mixture instead of the carb heat. Once. Never did that again! 

Fozzer wrote on Sep 14^th, 2009 at 6:16am:


I've had carb ice form in cruise over mountainous terrain when flying through foggy conditions a little bit above freezing. I noticed the engine sound had changed from it's cruise setting, like the throttle had slipped back. I pushed to full throttle and got little RPM change. This immediately set of the carb ice flag in my head. I pulled the carb heat and within 15 seconds the RPMs were coming back up again. If it is caught early, it is not really a big deal. The warning signs are subtle but unmistakable.

The danger is having the ice form in a high-workload situation when the signs are harder to notice. This is why carb ice in the pattern is of particular concern. With the pilot's attention divided and constant power changes, it is easy to miss the warning signs. This is why it is always a good idea to pull carb heat with power reduction. Even if ice doesn't form 999 out of 1,000 times, the one time it does form you could easily not notice until the engine stops when turning base.

Mtns2Skies wrote on Sep 19^th, 2009 at 6:58am:


I'm sure he's talking about turning pitot heat on after takeoff 

And I'm sure he meant VFR conditions, as in not IMC, as it would apply to pitot heat.