View Full Version : Mach .76 for Turbulence/Engine Anti-Ice ON
cameltoe2006
20th Dec 2006, 07:20
I have seen some pilots select Mach .76 everytime they feel some buffeting during cruise. What is really the purpose of selecting Mach .76?
I have also seen some pilots select the engine anti-ice ON even if the TAT is reading more than +10 when they anticipate that the aircraft is going inside clouds (even without precipitation and no visible moisture). When do you exactly need to turn the Engine Anti-Ice ON?
Jonty
20th Dec 2006, 08:04
You don't say what aircraft, but I imagine .76 is the turbulent air penetration speed. This is the speed the manufacturer says will give the best protection during turbulence, and when I say protection I mean between Mcrit and stick shaker. For the B757 we work on .78, it reduces gust loads on the airframe and can smooth out the ride for the pax.
The selecting of TAI above +10 has the effect of switching on the ignitors, or it may be because they think it will be required and gives them one less thing to think about. Try asking them next time they do it.
For the TAI on the B757 its when the aircraft is in cloud (vis 1 statute mile or less)or visible moisture and the TAT is above +10c and the OAT is above -40c (the lower oat limit does not apply when the thrust leavers are at idle)
Stand to be corrected on any of the above.
captainpaddy
20th Dec 2006, 11:34
Don't forget that the TAT can drop dramatically on entering cloud. Sometimes it will be put on in anticipation. I have seen a TAT of 15 degrees drop to 7 on entering a particularly dense cloud before.
Turning the Anti Ice on also increases the compressor stall margin.
Some normal Op's procedures a various Air Carriers call for A/I anytime visable moisture is encountered or expected at 10c and below.
The idea is to have the A/I on when you need it, I. E. - the phrase "expected". Obviously on modern transports, the A/I acts fairly quickly. However, you want it on to prevent Ice ingestion and not to de-ice. sO, ANTICIPATE the need for A/I.
On the DC-6 we would "fire up the heaters" at least a 1000' before penetrating anticapted Icing conditions.
The phrase "Anti-Ice" vs "DeIce" regulates how we use it.