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Windshield Heat

Old 12th Jan 2010, 01:48
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Windshield Heat

Asked a question about the use of windshield heat for an exam question, and the answers include,

1. Stop windshield fogging up on departure when climbing into warmer air,

2. Increasing strength of windshield incase of a bird strike.

Had a look through various technical notes and also aircraft flight manuals but can't find any reference to the bird strike bit.

Have a recollection of somebody mentioning that in some of the first generation jet's the winshield heat was turned on for that purpose.

Over to the PPRuNe knowledge bank.
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Old 12th Jan 2010, 02:14
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Answer 2, if that is the only two answers. But heat is also for defogging and deicing. Fly for two hours at cold temps at FL 330, descend into warm, humid air and the view out the front will be mostly white and very short. F/O forgot it once, we took a few extra vectors.

GF
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Old 12th Jan 2010, 02:26
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Windshield heat

Windshield heat is primarily to keep the windshield clear of external icing and internal fogging, however it also provides protection against impact damage from bird strikes and hail etc. For this reason most, if not all, commercial aircraft with heated windshields will have a speed restriction below 10000' if the windshield heat is inoperative, commonly not above 250 KIAS below 10000'.

Last edited by Old Fella; 12th Jan 2010 at 02:48.
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Old 12th Jan 2010, 04:00
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2. My old Boeing manuals (which I no longer have), referenced window heat as providing bird strike protection.
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Old 12th Jan 2010, 04:12
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As in most multi-choice questions, you need to sort out the nonsense first to eliminate one possible answer straight away. In a 3 or 4 choice question, one answer is always totally wrong, one may be almost right and you then need a degree in English comprehension to sort out the nuances of the other two. With experience, anyone with a broad knowledge of a subject, plus sound English comprehension and good reasoning can pass a multi-choice exam on a similar subject with little or no specific study. That is why systems knowledge seems to be declining , but I digress with my rant...
In a 2 choice question it is easier to arrive at what what they want. In the above, answer 1. is nonsense, because even if you did climb into warmer air, fogging would not be much of an issue. For air to be warmer above, you would be looking at an inversion situation and that would only be for a short part of any climb. In any case, in a type of aircraft likely to have windscreen heat you probably don't need to see outside to climb, only to land. So answer 2 is your only choice.
Bird impact protection is a major reason for windscreen heat on many transport types. The impact protection function of heat is evidenced by speed restrictions when it is unserviceable.
The prevention of ice formation or fogging is certainly a descent issue, but could be taken care of with external bleed air or alcohol spray etc and in some older types that's how it is done.
Fortunately the descent fogging answer was not offered, or depending on how it was worded, you could have had a difficult choice. The exam would then need to provide a 'both 2 & 3' type of answer (statistically it could probably be shown that whenever you get a 'both 2 & 3' choice, it is the correct answer on 90% of occasions).

Last edited by Mach E Avelli; 12th Jan 2010 at 08:59.
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Old 12th Jan 2010, 05:32
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Foggy bird strikes

From the Boeing 737NG System Handbook: “…. A conductive coating on the inner glass pane of Windows 4 and 5 permits electrical heating to prevent fogging.”

I don’t like the first option: “Stop windshield fogging up on departure when climbing into warmer air”. First of all on departure climbing you encounter cooler air, inversion exception noted, not warmer air. Secondly fogging up usually occurs after a cold soaked window is introduced into warmer moist air on descent. Information regarding windshield heat is usually found in the anti-ice system of the manuals it’s designed to keep the windows from icing over.

I was also under the impression heated windshields increase the glass’ ability to withstand a bird strike. But I can’t find that statement anywhere in the Boeing 737NG manual. The only limitation I could find in the QRH (Quick Reference Handbook) was not to excee 250 knots when below 10,000 feet for a window overheat light on condition. The speed limit was only below 10,000 after the window heat switch had been turned off. We are limited to 250 KIAS below 10,000 anyway.

If I had to choose between only those two choices I would choose the answer that links window heat to anti-ice function, but that is not given it’s “fogging” or “bird strikes”. Because my Boeing 737NG System Handbook does mention, “fogging” I would reluctantly have to go with the first option even though I don’t like it.

Last edited by Northbeach; 12th Jan 2010 at 05:59.
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Old 12th Jan 2010, 06:59
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Window heat

Window heat also ensures the screen vinyl layer is kept elastic and resilient and better to absorb damage in case of a bird strike - not so if the screen is cold and hard.

If the screen heating fails then there is always the forced fan air to reduce possibility of fogging.

TW
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Old 12th Jan 2010, 09:06
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Which reminds me. According to the certification standards, birds only fly up to 8000ft QNH. Many aircraft have speed restrictions below 8000ft based on the increased likelihood (apparently) of birdstrike. Obviously whoever made the rule didn't think it through very carefully or they would have worded the limitation to read '8000ft above surrounding terrain'.
Tell it to the birds.
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Old 12th Jan 2010, 09:54
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If my memory serves me correct the answer (because you're in aus and I'm assuming this is the aus ATPL) is with regards to birdstrike. It should however be noted that it doesn't "increase strength" rather it increases flexibility (as previously stated).

Here's why:
1. Stop windshield fogging up on departure when climbing into warmer air,
Condensation or "fog"
... condensation occurs when moist air comes into contact with cool surfaces, such as glass. This type of condensation appears when the dew point in the air is higher than the temperature of the glass.
Firstly, condensation typically wont form on the outside of the windscreen due to airflow. So window heat for demisting is generally to stop the inside of the window fogging up. This can only occur as the glass temperate drops due to a COOLER OAT as the aircraft climbs (as describe in the quote above).

If the aircraft is climbing into a warmer OAT one must assume the glass is still cold so yes condensation could form on the outside surface (again refer to above quote) of the glass; if this was possible, however due to airflow it is HIGHLY unlikely (I can not speak for all aircraft types however).

Few points:
How much warmer will the air get in an inversion, is it enough? (it can but read on for why its negated)
If the cabin temp is already at say 23/24 how long would glass (low latent heat) take to warm to ambient OAT? Especially if its warm on one side already.
How long until you're through the inversion back to the same OAT prior?
I know thats a little bit practical for a theory question but its a part of the reasoning.

Typically trick question.

PS not the greatest explination but its the best way I can word it.
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Old 12th Jan 2010, 11:37
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It's not tricky at all.
When was the last time anyone saw "fog" on the outside of a windshield?
As the OAT is increasing that's the only place it could form and I would suggest the wipers would be more effective in removing moisture on the outside if any were to form.
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Old 12th Jan 2010, 11:56
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I believe that the primary purpose of windshield heat is to deice the windshield - everything else is a secondary benefit.

I would not be so quick to dismiss the benefit of windshield heat for defogging purposes. When descending into hot, humid environments (for example, near the ocean in tropical countries), condensation often builds up rapidly on the inside of the windshield. Windshield heat is very effective in preventing (or removing) this condensation.

As for birdstrike protection - well, my Twin Otter has windshield heat, but there is no mention in the AFM for turning it on for bird impact protection. Might have something to do with the fact that the Vne is 170 knots. Besides, most Twin Otter birdstrikes occur on the rudder trim tab, when the bird is attempting to overtake the slow-moving Twin Otter.
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Old 12th Jan 2010, 12:52
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The electrically heated conductive coating on the outside of windows 1and 2 has the function defogging,and ice build up prevention. The conductive coating on the inside of windows 4,5 prevent fogging

The temperature controllers maintains windows 1, 2 at the correct temperature to ensure maximum strength in the event of a bird impact.

The MEL or QRH will direct you to adhere to the bird strike speed below FL 100with inop window heat. Some A/c, such as 757 have a lower limit e.g 313 kts below fl 80.

Boeing systems manual refers 3.20.2
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Old 13th Jan 2010, 02:27
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Window heating

Sorry Kirks gusset, the clear conductive coating on the window 1 & 2 is not on the outside of the window, but on the inside of the outer pane in the laminated window.
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