Hello,quick question. Why are the probe heat switches kept off during preflight? I understand they are AC powered, but the APU should be on by then regardless the GPU usage. I remember I found out why some time ago, but can't seem to recall it right now.Thank you.

I think its purely for saftey reasons, so if any one of the ground crew working on the aircraft accidently touch one of the probes they dont burn themselves.

I think its purely for saftey reasons, so if any one of the ground crew working on the aircraft accidently touch one of the probes they dont burn themselves.

thats what i thought too but working on military jets which have the same systems probe heats, for them to work the aircraft has to be in the air, there is a switch in the wheel well to tell it in the air. For problem solving on the ground we had to fool the aircraft by using a tong which had to be inserted in the switch in the wheel well. So was wondering if civilain jets are the same or not?

Peter: some have this mod, it is not standart on 737 at this time.

Just guessing but maybe they have it to lower maintenance costs. Or maybe so they won't overheat on ground.Manfred

They are what we call, meat leaving hot. Big safety issue. On some systems, this will decrease the life if left on for long periods of time with out air flowing over them. This can lead to distorting and damage which can lead to instrument errors.

on some planes like the 767 and 757 they come on when the engines are started

It varies by aircraft model. Many large transport category aircraft have a dual level pitot heating system. If turned on with weight on wheels, a low amount of voltage will be applied, enough to prevent ice on or in the tube if the air temp is below freezing, and snow or freezing rain is falling, but the amount of heating is less than what is required in flight. Airbuses have such a system, and I believe the NGX does as well.On takeoff, full voltage (max heat) is applied.Safety is a definite concern, as the tubes will reach a temperature of several hundred degrees if full voltage is applied on the ground with no cooling airflow. Enough to cause immediate third-degree burns if a person were to touch the probe. I had a friend who burned the palm of his hand so badly from grabbing the heated AOA vane on an MD-80, that he had to have skin grafts.There are many aircraft models that do not have a dual-level sysyem... on these aircraft, the pitot heat goes to full hot whenever it is switched on. Most small to mid-size business jet aircraft are like that. I once saw a plastic pitot tube cover sleeve actually catch on fire on a Hawker 800 jet, when the pitot heat accidentally got switched on in the hangar while the aircraft was powered up by a GPU!For those aircraft without a dual-level heating system, the life of the pitot tube will be drastically shortened if the heat is activated on the ground for any length of time when air temps are well above freezing. These aircraft will usually have a checklist indicating that pitot heat should only be turned on just before the plane takes the runway for takeoff, and switched off as soon as possible when exiting the runway after landing.

Thank you, crystal clear! :)

Other than for safety reasons, the probes becomes extremely hot, the generated heat, on ground, with no air flowing at high speed around them, will damage the probes.

They are left off during the standard preflight and on in the before taxi checklist. However, if you have freezing conditions (OAT 3 degrees celcius or below, vis below 1600m and precipitation of any kind) then put them on in the preflight.Henry Lidster

We never switch them on until just before we push back. Our new 737s actually have swich positions of AUTO and ON. In the AUTO positon they will automatically turn on when the engines are started and obviously ON just turns them on. Our procedure is to turn the switches on even if we are flying a plane with the auto feature. I don't know if it is a new standard feature or something we are ordering.One day in the F100 we were pushing back and the tug drivers headset cord touched the pitot tube while the heat was on. It melted all the way through almost instantly. Those things get seriously hot.

Hello,quick question. Why are the probe heat switches kept off during preflight? I understand they are AC powered, but the APU should be on by then regardless the GPU usage. I remember I found out why some time ago, but can't seem to recall it right now.Thank you.

To discourage US ground staff from heating their hotdog buns on them.Andrew Vincent

We never switch them on until just before we push back. Our new 737s actually have swich positions of AUTO and ON. In the AUTO positon they will automatically turn on when the engines are started and obviously ON just turns them on. Our procedure is to turn the switches on even if we are flying a plane with the auto feature. I don't know if it is a new standard feature or something we are ordering.One day in the F100 we were pushing back and the tug drivers headset cord touched the pitot tube while the heat was on. It melted all the way through almost instantly. Those things get seriously hot.

I think the AUTO/ON rather than OFF/ON options are the result of an NTSB recommendation, but that might have been a different type plane.

AUTO/ON is standard on our latest ones.

 

Henry Lidster