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TAAMGuy
26th Oct 2010, 15:01
Do the newer types (B777, B744, A330/340, A380) still do step climbs for the trans-oceanic hops? If so, what kind of restrictions would that bring into play? i.e. for a MTOW how low and how long until getting to the higher altitudes?

Intruder
26th Oct 2010, 15:32
Yes. Depends on the load and winds, but lowest altitudes are in the range of FL290 and range up to 430 for light loads. Depending on traffic, steps may be 1000' to 4000'.

TopBunk
26th Oct 2010, 17:45
For the B747-400 you intially burn 10-11 tonnes per hour in the cruise and the optimum altitude increases approx 1000ft per 10 tonnes burnt. So in NAT OTS you may ideally climb each hour (although in practise about each two hours).

In normal rvsm airspace, with 2000ft between levels in the same direction, it would be less frequent, although in either scenario, and subject to weather/turbulence predictions, sometimes you will decide to fly above optimum and close to maximum if you feel you may not 'get' the altitude later on.

It's all a game of strategy, just like coming out of Singapore pre-rvsm, when you may be offered FL240/260 (rather than your preferred FL310) or a 30 minute delay. You burn lots of fuel down at FL240 (which you may have to stay at until India) but will make it back up later on (or most of it with the RB211's) when you climb straight up to FL340/380 for the last 7 hours of flight.

TAAMGuy
26th Oct 2010, 17:52
So if I am understanding correctly, the aircraft is capable of higher cruise altitudes but the crew stays lower to acheive the 'optimum' level, which ascends by 1000' for every 10 tonnes of fuel burn? :rolleyes:

CabinMaster
26th Oct 2010, 20:13
No, you do not understand correctly.

Cruise altitude is limited by aircraft performance. The "optimal" cruise altitude (lowest fuel burn) is usually the highest possible.

Cruise altitude is also restricted by air traffic control. Today the airspace is separated into levels of 2000ft (in one direction). So an aircraft can climb from 31000ft to 33000ft as soon as it has
> the ability (lost some weight)
> the permission (by ATC)
The initial altitude depends on many factors (weight, number of engines, weather) and usually between 29k (B747-400 at max weight) and 37k (B737-700 on a short flight).

As people explained, there may be circumstances where you refuse a higher (= better) altitude. Weather may be one, as the turbulence margin shrinks with altitude.

parabellum
27th Oct 2010, 10:14
The "optimal" cruise altitude (lowest fuel burn) is usually the highest possible.


This was not the case on the B744 with the PW4056 engine that I flew, but may well vary according to the regulatory authority and the buffet margin each different authority requires.

Wizofoz
27th Oct 2010, 11:35
Optimum altitude is not usually EVER at maximum altitude.

How far below varies not only with aircraft type/engine combinatin but also with conditions on the day (Increasing headwind with altitude for instance, will mean it is lower than nil wind.)

As an example, Optimum altitude in a RR powered 777 is usually well below maximum altitude, while it is close to max altitube in a GE powered one.

TAAMGuy
27th Oct 2010, 12:45
OK, I think that I'm getting it now. Using the B744 numbers given, in RVSM airspace, burning 10-11 tonnes/hr you could climb to another level (2000') every two hours. This could mean a number of altitude changes during say a JFK-EGLL trip.

I have some archived flight data, and after questioning a controller who worked Gander Oceanic, he says that by time the flights were passed from New York ARTCC, they were all level at their final altitudes. This, based on previous posts, would this be due to the aircraft NOT being at their MTOW when departing? So, for the longer range types (ER's) are the tanks always full for a transatlantic flight or just enough fuel to satisfy the flight planning requirements? Can I deduce that if a B777-300ER can fly for 15 hours then for a 6-8 hour flight eastbound over the atlantic, the tanks are maybe half full????

SloppyJoe
27th Oct 2010, 13:51
you only take enough fuel to satisfy flight planning with an occasional extra bit if expect a delay or extra miles on route. I would say 80%+ of the time it is just enough to satisfy flight planning.

Luckyguy
27th Oct 2010, 16:06
TAAMGuy

Simplified: You also need to understand that although your friendly Oceanic Controller (through a Radio Operator) may have told you that the jets are at their final level after release from NY, that may only be the final NAT OTS level, due to traffic in front, above, behind and below. All a/c using the NAT OTS are separated procedurally (in lateral, stream and vertical plane) If some other guy is 6 minutes ahead and 1000' above you and you want to climb 1 level (1000'), it ain't gonna happen unless he's faster and until the controller can assure 10 minutes streamed separation. Level changes are reasonably common, depending on traffic. Flights coming off the Atlantic and going further than Dublin/Glasgow/Manch etc will probably be requesting a climb towards optimum FL for the remainder of the trip.

All of the above comments re: FMS optimum levels/Authority requirements/buffet margins etc are all taken into account too.

Ball park: B737-800 using default 5% CRZ CG at about 70000KG gives you an increase in optimum altitude of 100 feet every 9 (ish) mins, using 42 Kg/min fuel burn.

TopBunk
27th Oct 2010, 17:09
I have some archived flight data, and after questioning a controller who worked Gander Oceanic, he says that by time the flights were passed from New York ARTCC, they were all level at their final altitudes.

TAAMguy

The Oceanic guys issue the clearances to the flights well in advance of them reaching the oceanic boundary. The 'domestic' controllers and the pilots then between them, negotiate when any climb or descent will be carried out in order to present to the 'oceanic' controllers traffic in accordance with the clearance prior to the boundary.

In that sense, yes, the flights are at their final altitudes.

That, however, does not mean that they remain at that altitude for the duration of the crossing.

Do not forget either, that in NAT airspace normally on a given track the traffic is all in one direction, therefore separated vertically only by 1000ft rather than the rvsm 2000ft with bi-directional traffic, so the option exists for climbs of only 1000ft. In reality, it is unusual to have more than 1 step climb on a crossing.

nmcpilot
1st Nov 2010, 16:49
So does WAT limit come into this equation? Or is WAT limit used for performance on the ground only or through all phases of flight?

If we take various limitations of other traffic and ATC out of the equation for now..

My way of thinking was that an aircraft will climb to the altitude it gets its optimum fuel burn for that weight as calculated by the FMS taking into account various cost index inputs calculated for the day? To get the optimum SFC for the speed that it wants to fly at... So say for example an aircraft initially climbed upto FL290 because basically the FMS would have calculated that for the weight the aircraft was at that would get the speed it wanted with the most efficient SFC?

So it would not request a further climb to say FL330 until it had burned more fuel and lost more weight because if it just climbed straight up to FL330 due to the fact it is fairly heavy it would require more thrust in order to maintain straight and level than if it just stayed at FL290.. Therefore reducing the SFC? Am I understanding correctly so far?

But then considerations would have to be taken into account to the winds inthe upper atmosphere and also the temperature. So the height of the tropopause would have to be considered as to where the optimum temperature would be?

I'm not looking to get slated here just trying to learn to see if I am kind of in the right area with my way of thinking?

This is why I ask does WAT limit get considered in all phases of flight? I assume that yes it does but wasn't sure. As obviously it is very critical in take off performance if you are taking off on a shorter runway on a hot day with a high density altitude with a heavy aircraft.

I know various ATC limitations have to also be implemented but just asking from a pure performance point of view imagining there were no other aircraft in the sky.

TopBunk
1st Nov 2010, 18:11
MMCPilot

See here http://www.pprune.org/tech-log/326819-t-o-landing-climb-limit-weights.html

WAT limits affect aircraft in all phases of flight, but in the sense you are referring to, not in the cruise phase (or are not known as such, at least).

WAT limits generally refer to the take off, climb and landing phases.

As far as the cruise is concerned, basically the weight/mass will determine the optimum and maximum cruise levels but the real best performance will be determined in addition by the environmental (temperature, weather and wind), regulatory (levels available for the direction of flight) and strategic (forward planning vs other traffic).

The tropopause itself is not a factor other than the temperature implications.

From a theoretical standpoint, the optimal flight would be allowed a block of altitudes in which to operate (as Concorde often was) and would drift up and down as weight, wind and temperature caried. From a practical standpoint this is only available in a few, low aircraft density situations.

nmcpilot
2nd Nov 2010, 00:33
Ahh thanks for that TAAM =)