BoeingGuy From United States of America, joined Dec 2010, 2376 posts, RR: 7 Reply 2, posted (1 year 1 week 4 days 14 hours ago) and read 3657 times:
THR REF is an autothrottle mode in which you target a specific EPR or N1 (as opposed to a certain speed or climb rate). The previous poster accurately describes it also. For takeoff and climbout, the autothrottle is in the THR REF mode in which it's setting the calculated takeoff and climb thrusts (e.g. N1 of exactly 99).
The way I used to try to explain it in layman's terms is suppose you are driving a car. Instead of pushing on the gas pedal to go exactly 60mph, you are saying "give me exactly 77 horsepower". That's an analogy to how THR REF works. Other autothrottle modes in other phases of flight will set thrust to hold a speed or specific climb or descent rate.
phunc From United Kingdom, joined Mar 2011, 69 posts, RR: 0 Reply 3, posted (1 year 1 week 4 days 12 hours ago) and read 3576 times:
Ok thanks guys, but how does that differ from assumed temp take off (flex for the Airbus)? My understanding is that, for example, assumed temp of say 45C would give max power in a 45C environment (so less than full available to protect the engines). Are the two linked?
BoeingGuy From United States of America, joined Dec 2010, 2376 posts, RR: 7 Reply 4, posted (1 year 1 week 4 days 11 hours ago) and read 3560 times:
The thrust reference is the actual thrust calculated and used for takeoff.
The Assumed Temperature Derate, if selected, is used to calculate the takeoff thrust. Perhaps the pilot selects an assumed temperate of 35C (say it's 15C outside). Then the Thrust Management Computing Function (TMCF) in the 777 calculates what thrust that is. For example, an assumed temperature selection of 35C may result in 99.9 N1. Full underated power may have been 103 N1.
So in this example, the THR REF value would be the result of whatever Assumed Temperature Derate was selected. You can also do un-derated full takeoff power, or another method called Fixed Derates (10% and 20% reductions, usually).
In other words, the THR REF value is the result of what assumed temperature was selected.
There are several ways you can select what the THR REF value will be for takeoff - either manually or the computer calculates it for you:
1) Full un-derated takeoff power
2) Select an Assumed Temperature Derate
3) Select a Fixed Derate (TO1 or TO2)
4) Select a combination of an Assumed Temperature Derate on top of a Fixed Derate (say TO1 + an assumed temp)
5) Select a Takeoff Bump on those airplanes so equipped (this is a special higher than normal thrust, opposite of a derate)
6) Manually select a THR REF in the CDU. For example, you can just type in the takeoff thrust, say 99.9 rather than have the TMCF calculate it for you
7) Uplink a THR REF to the TMCF by datalink (a.k.a. ACARS).
Not all airplanes can do all of these. If you do #6 or #7, presumably there's a dispatcher figuring all this stuff out for you. Pilots don't just make up a takeoff THR REF value.
You can derate climb thrust too, but only fixed climb derates (CLB1 and CLB2).