According to this page on the 737's operational limitations, the Original 737s (737-100/-200) are limited to 250 KIAS below 10 kilofeet if the windscreen-heating system is inoperative:

Anti-Ice & Rain




Window heat inop: max speed 250kts below 10,000ft. [My emphasis.]

Why is this?

  • 19
    $\begingroup$ "Kilofeet" is my new favorite term. $\endgroup$ Commented Oct 14, 2019 at 12:48
  • 6
    $\begingroup$ Coincidentally, there is a more general 250-kt speed restriction below 10,000 feet: aviation.stackexchange.com/questions/20837/… $\endgroup$
    – David K
    Commented Oct 14, 2019 at 15:31
  • 1
    $\begingroup$ @DavidK Yes, most countries have such a restriction, but it is not a universal worldwide rule. $\endgroup$
    – Bianfable
    Commented Oct 14, 2019 at 15:54
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    $\begingroup$ @Bianfable Good point, and apparently even when there is a restriction ATC can grant a clearance for a higher speed. In any case, if it is physically unsafe for the aircraft to fly faster than 250 kt below 10,000 feet, that is something I would want to know regardless of any other speed restrictions. $\endgroup$
    – David K
    Commented Oct 14, 2019 at 16:27
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    $\begingroup$ It's cold outside. $\endgroup$
    – Mast
    Commented Oct 14, 2019 at 19:55

1 Answer 1


To reduce damage in case of a bird strike.

The restriction is not only for the 737-100 and -200 models, the 737 NG QRH says:


In flight:

WINDOWS HEAT switch (affected window) ..... OFF

Limit airspeed to 250 knots maximum below 10,000 feet.

Pull both WINDSHIELD AIR controls. This vents conditioned air to the inside of the windshield for defogging.

The reason for restricting the speed is that the windows are less elastic at lower temperatures and can therefore not absorb a bird strike as well as at nominal temperature. Since most bird strikes occur below 10,000 feet, the restriction only applies there.

I could not find a good source for this, but it is e.g. discussed on pprune.org:

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.

and airliners.net:

cold windows become very brittle whereas warm windows have a degree of elasticity in them that makes them withstand birdstrikes better.

A new window was developed for the 737 and is installed on all models built since mid-September 2010. The technical data by PPG Aerospace list the following among the improvements:

Elasticity at low temperatures reduces potential for delamination and cold chipping

So low temperature elasticity seems to be improved. I do not know, if they removed the restriction from the QRH for the newer models.

Airbus also does not seem to have such a restriction. From pprune.org:

When I asked Airbus exactly this question their reply was that the windscreens had been tested at 350+ knots and cold.....therefore no problem!

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    $\begingroup$ My friend once told me that in the winter he thew a TINY pebble, gently and it landed on a car windscreen and shattered the whole thing. I wonder if this is why... $\endgroup$
    – Cloud
    Commented Oct 14, 2019 at 15:45
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    $\begingroup$ @Cloud I'd say partly, plus bad/old window with possibly invisible other weakening (like chassis pressuring a point), plus possibly harder throw than remembered/admitted. $\endgroup$ Commented Oct 15, 2019 at 12:32
  • $\begingroup$ Since most bird strikes occur below 10,000 feet "!!!!" Birds only fly to a couple hundred feet, but, I only now learned a few birds fly to astounding altitude: wikipedia.org/wiki/Rüppell's_vulture $\endgroup$
    – Fattie
    Commented Oct 15, 2019 at 17:56
  • $\begingroup$ @Cloud. Shattering glass with tiny pebbles is a thing, assuming it's the right kind of pebble. elitereaders.com/car-thieves-ceramic-stones-break-car-windows $\endgroup$ Commented Oct 17, 2019 at 15:56
  • $\begingroup$ @user3445853: Plus possibly bigger "pebble" than remembered/admitted. $\endgroup$
    – Vikki
    Commented Jan 31, 2020 at 1:06

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