FR B738 - why such a high alt. for a short trip?

Comhrá

Just watching FR 108 (Snn-Stn) climbing to FL390 outbound. Why would they want to climb so high on a short leg? Lower fuel burn?

Cessna_Pilot

Lower fuel burn the higher the altitude, strong upper winds to name but a few reasons. SNN to STN's not the shortest leg either. They're only over the Wexford coast there and already at cruise altitude.

Comhrá

Cessna_Pilot wrote: »

They're only over the Wexford coast there and already at cruise altitude.

I was watching them climb and they made fl 330 in just over eight mins. Pretty impressive climb rate!

bronn

Huh. Never knew that there was less fuel burn at high altitudes. I've frequently caught EI610 from DUB to AMS. A good few times, it's been delayed on departure. Last time I took it, it was sheduled to leave at the usual 17:10 but didn't actually depart until 17:45. We rocketed up and in under an hour, we were in AMS and made the scheduled arrival at 19:40 (AMS time). It always flies up to 39,000ft and starts the decent into AMS just off the Kent coast. Never fails to make my ears pop like crazy either. If I have a connecting flight in AMS, I've stopped freaking out that EI610 won't make it. Up to now, I assumed it had to fly that high to avoid Heathrow traffic. As it quite typically gets delayed in DUB, I have wondered why they never adjust the schedule. Makes sense now - fly high, use the wind and burn less fuel.

shedweller

Is there much more fuel used during the steep climb, compared to a normal climb? Is the overall fuel use over the whole flight much less than if a normal climb was done?

Jack1985

bronn wrote: »

Huh. Never knew that there was less fuel burn at high altitudes. I've frequently caught EI610 from DUB to AMS. A good few times, it's been delayed on departure. Last time I took it, it was sheduled to leave at the usual 17:10 but didn't actually depart until 17:45. We rocketed up and in under an hour, we were in AMS and made the scheduled arrival at 19:40 (AMS time). It always flies up to 39,000ft and starts the decent into AMS just off the Kent coast. Never fails to make my ears pop like crazy either. If I have a connecting flight in AMS, I've stopped freaking out that EI610 won't make it. Up to now, I assumed it had to fly that high to avoid Heathrow traffic. As it quite typically gets delayed in DUB, I have wondered why they never adjust the schedule. Makes sense now - fly high, use the wind and burn less fuel.

While higher altitudes may mean lower fuel burn (in some cases where winds permit), flights are not guaranteed a higher altitude always at the same time - The altitude is a lot of the time restrained by eurocontrol particularly to slot controlled airports, although pilots obviously can request higher from the atco sector they are in. Also winds can be higher/lower at certain altitudes i.e. at FL350 you could have a headwind of 100kts and at FL370 a headwind at 75kts and vice-versa, FL370 in that case would give you a better trip time.

Is there much more fuel used during the steep climb, compared to a normal climb? Is the overall fuel use over the whole flight much less than if a normal climb was done?

I would assume you mean step climb? And generally yes - You'll burn more during a step climb against a continuous climb (although in certain cases that can actually vary), as the air-density may be a lot higher at your stepped climb initial level against your cruise level. The climb usually burns the most fuel in any trip.

urajoke

Jack1985 wrote: »

While higher altitudes may mean lower fuel burn (in some cases where winds permit), flights are not guaranteed a higher altitude always at the same time - The altitude is a lot of the time restrained by eurocontrol particularly to slot controlled airports, although pilots obviously can request higher from the atco sector they are in. Also winds can be higher/lower at certain altitudes i.e. at FL350 you could have a headwind of 100kts and at FL370 a headwind at 75kts and vice-versa, FL370 in that case would give you a better trip time.



I would assume you mean step climb? And generally yes - You'll burn more during a step climb against a continuous climb (although in certain cases that can actually vary), as the air-density may be a lot higher at your stepped climb initial level against your cruise level. The climb usually burns the most fuel in any trip.

It's the local air traffic provider via Eurocontrol who impose level restrictions. This is done either to reduce the number of conflicts caused by aircraft climbing up through overflying heavies at say 320-330-340 to get to 370-380-390 and then descending back down through them again or because of an over capacity caused by a high demand from traffic this can be caused by heavies routing to NAT tracks or delays causing a glut of traffic at a particular time.

I think he does mean steep climb. The aircraft uses the same amount of power but if the aircraft is lighter it can get to cruise level quicker whereas with a heavier aircraft it takes longer to get to cruise level using more fuel. Getting to cruise level quicker gets you up to the more efficient levels and you can be there for longer using less fuel, also being higher means you descend for longer using less thrust ergo less fuel for longer.

bkehoe

The flight plans on the day will be calculated for the optimal level. Many times we'd be light enough to go all the way to 410 but the flight plan wants us at 350 for example as often jet stream cores are at around 300 - 350 around here, so if its a tailwind component then the higher fuel burn will be more than offset by the higher ground speed. Of course going the other way one would want to be as high as possible to avoid the wind.
Climb speeds are also dynamic depending on the wind one tells the FMC. Depending on the Top of Climb wind it will select a higher or lower climb airspeed (lower speed means a higher rate of climb thus reaching cruise faster) if it wants to quickly take advantage of a large tailwind for example.

Comhrá

Great posts guys, thanks.

As a matter of interest, what would be the max. rate of initial climb - say up to 10,000 ft. for a B738 with an average load and average conditions?

bronn

Yeah, really great posts, lads. Thanks a mil. Very interesting.

Oscar Bravo

Tonights BRS-DUB flight (EI-EVS) is flying at a fairly low FL240 for the cruise.

arubex

Back in the 1970s there was a BCal route from London ( Stansted I think ) to Southampton that regularly used the ability of the One-Eleven to climb directly to cruising altitude at MTOW; and once it hit FL350 it was literally time to descend!

Not sure how much of that was done for fun versus fuel efficiency.

Nowadays everything is flown according to the Cost Index dialled-in to the instructions of the accountants.

de biz

Oscar Bravo wrote: »

Tonights BRS-DUB flight (EI-EVS) is flying at a fairly low FL240 for the cruise.

Its only approx 193nm so 240 is typical on that sector.