Speed window differences on Boeings
 

I have noticed on the last couple of types that I have flown that regardless of altitude or speed, when I open the window, the present airspeed in knots is displayed. Of course, I can press a separate button and it switches over to the present mach number but I remember wondering why the window opened as an indicated speed when I first saw this. I assumed that the designers had their reasoning for the design and that was that.

So I was surprised to learn while reading a couple of incidents on the 737-800 that above a certain altitude, the window will open to the present mach number. And not only that but if climbing with the window open with knots being indicated, there will be an automatic switch over to mach approaching FL 300.

Anybody know the reason for the difference and which design might be better?

https://view.officeapps.live.com/op/...041_final.docx

https://www.atsb.gov.au/publications...r/ao-2014-179/

I always thought it was dangerous to open windows on airliners in flight...

PDR

It is technically known as the IAS/Mach window.

Yes, I can confirm that is the design behaviour on the B737.

Why would you want it any other way?

However, disappointingly, the IAS/Mach changeover doesn't occur approaching FL300. I wish it did.

It occurs around FL260. This means it will change from an "appropriate" climb speed of say 280 KIAS, to an "inappropriate" climb mach of M0.69 or thereabouts. Which, if let continue unnoticed, will cause considerable embarrassment at higher levels.

Edit: I just read your first link. My point exactly. This is just one of those 737-isms that keeps us on our toes.

It probably defaults to what your limiting speed is. Lower altitude is knots. Higher altitude is Mach. From that perspective it makes sense.

Knot/Mach limiting crossover typically occurs in the mid 20's.

No, it doesn't. Read my post.

It appears that Boeing decided to want it another way as the 747 and 777 never automatically switch to Mach and the window never opens in Mach.

I was curious as to why.

JammedStab:

Not strictly true in your post number 7, with regard to the 777 IAS/Mach window automatically switching, according to this (albeit old) FCOM 1 copy:

Thanks Daxon,

You are correct. The improvement, I suppose is making it switch over at a much higher mach value. Notice that the FCOM mentions only during climb/descent. I will try to open it at the higher speed in climb/descent/cruise and see what happens as I have never seen it open to anything other than indicated airspeed in knots.

OK, I opened the window above mach 0.84 while in cruise and it was in knots. It appears that the information posted by Daxon correct in that nothing is mentioned about a switchover to mach in level flight.

Interesting. On the B737 it will open in Mach in the same scenario.