Britannia High-Lo Flight Regime
 

Just reading an article about the Bristol Britannia with the Proteus turboprop engine which turned out to have issues with engine icing flameouts. Unlike more modern issues we have seen on certain types leading to the requirement of increased lateral margins, the Britannia crews managed this with what was called a High-Lo flight regime. While one gets the general idea of what was done based on the name of the procedure, I am curious if anyone knows any of the details about this.

My understanding is initially flew around 8000ft or so, high fuel burn but decent speed then once weight reduced climbed to the high 20,s hopefully above the icing cloud!
I am sure some other posters will confirm or correct me.

This probably should be in aviation and nostalgia.
Brakedwell (ex RAF Brit pilot) likely to reply

The critical temperature range for engine icing was between indicated temperatures of +2 and +12 deg C. So if significant cloud was forecast, we used to start at a height where we would be warmer than +12 - maybe 10000 ft in the tropics. When we had burnt enough fuel off we climbed straight up to where the temperature was colder than +2 - maybe 22000 ft.

Without referring to "Not Much of an Engineer" by Stanley Hooker I can't remember the exact figures, but I think he said something like with a marginal change in cruising altitude, there wasn't a problem, but BOAC milked it for all it was worth.
It didn't help that the Proteus was basically a dog of an engine, although less so with the later versions.

It was a long time ago, but I agree with WID62's settings for the RAF Brits. I think these engine ant-icing valves were called the Fore Skin Jets! However the civilian Brits were a bit different. IOAT was +12 to 0 degrees when the B Skin Jets had to be switched on. I seem to remember the engine ant-icing system worked a bit better on the Civil Britannias.

I've downloaded the RAF and BOAC fliying manuals for the Brit. When i get a chance i'll take a look

+2c to +12c
 

In the RAF aircraft, the overall critical temps for the anti-icing to go on was between +2 to +12C. There was a further stricter restriction which applied if the temp was in the +2 to +6 range. My memory from 46 years ago must have offloaded some of the detail but it may have required a speed restriction or possibly another level of anti icing kit being switched on. Does any of this prod Brakedwells memory? As I recall we slowed down so that the kinetic heating reduced and the OAT then fell below the +2c critical figure. If it were then say, -1C you didn't need the anticing on. ....... so you took it off and promptly accelerated back into the +2 to +6C range......Catch 22.

Not much help I'm afraid, but nostalgic to recall a job I'd have had without pay. The saddest day of my life when they axed the aircraft.

Jetset 88.
Yes, your memory serves you well. The speed we slowed to was 200 kts, and yes that often took the temperature out of the critical range. Also, when it was a little warmer, say +7 deg, and you entered cloud, you put the icing on. This slowed you a little, the temperature dropped to +6 deg and you had to slow to 200 kts! It must be quite rare for a crew of any aircraft to be so reliant on a tiny needle movement on a small gauge.

Engine icing flameouts were common when I was with 99 Sqn in the early 1960's.
I posted here on PPRuNe some photos I took when we flew out to Entebbe with replacement engines for a Brit that suffered a double engine failure over Africa.
See here: https://www.pprune.org/aviation-hist...ml#post6487892