Poadrim From Norway, joined Oct 2008, 174 posts, RR: 0 Posted (4 years 9 months 1 week 3 days 22 hours ago) and read 8734 times:
I was thinking about fuel/operating costs (for some unknown reason...) and one thing got me thinking.
A 747 flies LHR-SFO and first cruise altitude is 32000ft and then climb to, say, 38000ft.
When programing the FMC (if I got it right!) The computer asks for a optimum cruise altitude(?) but you start at 32000ft does the pilots tune in 38000ft in with the autopilot from the start or must they change as they go?
Does this make sense?
Good judgment comes from experience. Good experience comes from someone else's bad judgment.
arniepie From Belgium, joined Aug 2005, 1292 posts, RR: 1
Reply 1, posted (4 years 9 months 1 week 3 days 22 hours ago) and read 8717 times:
I don't know 100% sure but I'm pretty certain that the FMC gives both an optimum altitude ,which offcourse will steadily increase during flight because the plane gets lighter (depending on atmospheric conditions and CI) , and a Maximum altitude.
But The pilot has to eventually dial in the height (below MAX ALT) in the FMC according to his flight plan and ultimately the height he is directed to by ATC.
If in the course of the flight they want to go to a higher/lower altitude they always have to get clearance first from ATC, afterwards they can go and fly to this new , usually higher, Altitude, the plane will not do it in itself , the pilots have to dial in a new altitude on the MCP(?) and I believe also in one of the pages on the FMC (eg, vnav-page).
Max Q From United States of America, joined May 2001, 6312 posts, RR: 21
Reply 2, posted (4 years 9 months 1 week 3 days 21 hours ago) and read 8709 times:
The Flight Management Computer (FMC) will be loaded by the Pilots with the Aircrafts zero fuel weight, it automatically calculates the gross weight by adding the fuel on board.
With this information loaded the enroute winds (also loaded in the FMC) will also be considered.
On the 757 and 767 that I fly the computer will then give you three altitudes to consider Max, Optimum and Recommended.
The FMC also considers the outside air temperature, if it is warmer than planned, your altitude capability is lower and vice versa.
At the beginning of the flight, as you mentioned, you will be heavy and your optimum altitude will be lower, as you burn off fuel your optimum altitude will increase as you get lighter.
So, ideally you want to try to stay as close to optimum as possible, but !
There are other considerations, for example westbound into the jetstream it may be more economical to stay lower, or sometimes higher than optimum to try to stay out of the worst headwinds, obviously it's no good being at optimum altitude burning the least amount of fuel per hour but then staying in the air an extra hour or more because your groundspeed is so slow and burning more fuel as a result !
Along the same lines it may be advantageous to select an altitude other than optimum to take advantage of a good tailwind (less time in the air = less fuel burned again)
So optimum is a 'still air value'
The 'Recommended' altitude in the FMC does consider these winds and adjusts it's, well, recommendation accordingly.
Other considerations that must be planned are, for example on the North Atlantic, you will usually (not always) have to fly on one of the daily established tracks and will be assigned an altitude. This may be significantly different than what you had planned for and you may burn more fuel as a result.
The altitudes you expect to fly can be pre inserted into the FMC for different points along the route, and it calculates a reasonably accurate prediction of arrival fuel based on this but it needs accurate information, plans and clearances can change affecting this drastically !
Incidentally, the computer (FMC) does not ask the Pilot for an optimum cruise altitude, once it has the Aircraft weight it tells him or her what that is.
Finally, the altitude you are cleared to fly will be set in the MCP (mode control panel on the glareshield) and the autopilot, if engaged will level the Aircraft automatically at this altitude (if flying manually the Pilot will level it off themselves)
As you are progressively cleared higher you will change your cleared altitude in the altitude alert display of the MCP.
You would not set the highest altitude you expect to fly.
Hope that makes sense !
The best contribution to safety is a competent Pilot.
KAUSpilot From United States of America, joined Jan 2002, 1976 posts, RR: 30
Reply 3, posted (4 years 9 months 1 week 3 days 19 hours ago) and read 8656 times:
We get a paper "flight plan" from dispatch before the flight which tells us when and where to climb and descend in order to operate with maximum efficiency. The flight plan takes many things into account, but mainly wind, temperature, weight, center of gravity, and aircraft performance. The FMC provides a double check through which the pilots verify that the airplane is capable of performing the climb that the flight plane calls for. The flight plan and FMC do not always agree on optimum altitude, but my company wishes the plane to be flown as closely as practical to the paper flight plan, so that's what most crews do. The most important thing is not to try and climb above the "maximum altitude" that the FMC gives you, as demonstrated by the crew of Pinnacle flight 3701.
How is the climb executed?
During preflight, you can program a "step climb" into the FMC at various points along the route. Most pilots look at their flight plan and program step climbs into the FMC at the appropriate places during the preflight process. When the plane reaches such a point during the flight, the crew calls ATC and asks for permission to climb. If permission is granted, the flying pilot resets the altitude selector on the flight guidance control panel to the new altitude. He or she then selects a vertical mode which result in the aircraft changing altitdue. I typically select a vertical speed below 1000 fpm since these changes are usually made in RVSM airspace near other traffic, and rapid climbs have the potential to trigger TCAS warnings. Limiting the climb rate also minimize rapid and sudden pitch or power changes. So no, it is not a fully automated process, there is still some pilot action required, at least on the aircraft I fly (B-744).
Could the FMC be programmed and autopilot be managed in such a way that the airplane would climb automatically at certain points along the route? Yes, it could. However, this is not normally done because other traffic is an unknown variable during preflight and ATC clearances are not usually issued this way.