Flight over water speed

[Squealing Pig]

Some aircraft have a flight over water speed specified...Why ?

There will be a logical reason for this but I'm dammed if I can work it out.

[Fright Level]

60 mins from nearest airport calcs?

Rod

[wysiwyg]

Seem to remember something about flight over water speed being the one used to calculate the acceptable range for a take off alternate.

[Squealing Pig]

Just bringing it back to the top....any more ideas...anyone...Genghis

[411A]

Which aircraft. Need more info....

[mcdhu]

My memories of 'overwater speed' are a little dim due to the passage of time and the ravages of sex, drugs and alcohol! I recall, however, that the HS748 Flight Manual quoted the overwater speed as 240kts TAS. This was the speed at which the calculations were done when it was deemed, for instance, that the ac must be within 60mins of a usable airfield at all times in case of engine failure - ie 240nms.
The overwater speed was an academic speed, in that the good old 748 could barely raise 240kts TAS on a good day with 2 engines operating, but 240 was the legal figure to be used in such calcs.
These days, we don't carry the FM itself, but extracts which are deemed to be sufficient when our manuals are agreed by the 'Authority', so we never see the present day equivalent - if there is one. But the A320, for instance, must remain within 400nm of a useable airfield so my guess is that the overwater speed has been set by the regulatory authorities at 400kts TAS.
On the other hand, I could be talking rubbish!!
Cheers
mcdhu
PS Having looked at my Ops Man Gen, it might be that they have changed from using a standard speed, to using the lower of TAS at Vmo or the achievable TAS. Will do more research and re-post if the thread is still active.

[4Screwaircrew]

In light aircraft it is my understanding that the overwater speed is the TAS used when calculating radius of search for a missing aircraft.