Step climbs

[xfeed]

I am trying to understand step climbs better. What I have read is you generally start 1000 or 2000 feet higher than optimum (as stated by charts or FMC), then when you are 2000 feet below optimum you climb 4000feet. All this is assuming ATC clearance has allowed this to occur.

So, leaving ATC to the side, my questions are, first, is this theory (called bracketing I believe) correct?

Second, lets assume that once in the initial climb the FMC states your weight puts you at 360 as optimum (payload is lighter due to less pax but the flight is a North Atlantic crossing of 3600nm). Would you climb to FL370/380 that early on? The assumed plane is a B772.

The flight plans that I see (they are real world) seem to indicate step climbs of 2000 feet intervals, not 4000 feet. Any light that any of you may shed for me would be greatly appreciated.

Cheers!

[oceancrosser]

Nowadays with what I fly (757/767) we try to stay pretty close to optimum all the time, climbing either 1000 or 2000 feet at a time depending on the area. In the North-Atlantic RVSM airspace either on or off the NAT tracks you may get 1000' climbs, but once in domestic RVSM airspace like over Canada, they are pretty strict on flying odd/even levels according to direction of flight.

Then again, hunting tailwind or more likely avoiding the worst headwind, you would disregard optimum for greater wind difference vs. FL

cheers,

[icemanalgeria]

There are 101 ways to fly the aircraft so I can only add my personal way.

When flying the North Atlantic we are flying in RVSM airspace so we don't need to climb in 4000ft blocks.

I personally use 500 below opt for a 1000 climb, 1000 below for a 2000 ft climb. ( take the mid point).

I use 1.5 hours per 1000feet to calculate the estimates for my next level change (9 minutes per 100 feet, although 8 minutes is closer the truth,using 9 minutes makes the maths easy)

When in the cruise I make a grid on my LOG like this.

Opt Max time Waypoint
310 330 1200 20W
320 340 1330 35w
330 350 1500 50w

I give my WAH (When Able Higher) requests based on Max if there is no Turbulence forcast but request my optimum.

Hope this helps, Just my way of doing things

[xfeed]

Excellent replies from both! Thanks very much for explaining the procedure. This was very helpful.

Cheers!

[JW411]

Another good rule is not to climb 2000 ft unless you are going to be up there for at least 1 hour otherwise the fuel you burn getting up there will exceed the saving.

This would normally apply towards the end of a flight.

Forty years ago, when ATC conditions permitted, we used to use a climb - cruise technique. In other words, we never stopped climbing until we reached optimum as weight reduced or else it was time to come down again.

From the ATC point of view, we were given a block clearance of 2000 ft or so. In other words, the space between F250 and F270 was ours.

This would not of course be possible nowadays but it was a very efficient way of operating.

[tournesol]

Among other things, the CRZ Mach No or Cost Index, will change your optimum level. Assuming these factors are constant, and only the performence is the criteria, then the optimum level is just that. You may however climb 2000 Ft above optimim, but you have to be aware that your buffet margin is now 1.3 g instead of 1.5 g. If no turbulence is forcasted, you should be ok.
One should avoid to climb above optimum level if shear ratio is on the high side, i.e turbulence forcasted.
I just hope I did not cause more confusion on the subject that is very little understood even among pros.
cheers.

[Intruder]

Rather than blindly going +2,000 or -2,000 feet (or whatever is required in RVSM) from optimum, use your wind forecasts and tradeoff tables to figure out when you should climb. Depending on the specific airplane, altitude range, and wind situation, staying low may actually use less fuel.

[justcoolen]

Just out of curiosity, how much are you in contact with your company dispatchers? During a busy day there are over 1200 oceanic crossings in a 24period. Depending on your company, there could be several company flights on your same track, within a few flight levels with very accurate information regarding winds, turbulence etc. Does CPDLC or Datalink provide dispatch with more accurate information?

About the odd/even levels in Canada. Some mathematician used a formula to develop a target level of safety. This mathematician says it is unsafe to use certain levels during a certain time of day. i.e. during the East or West bound flow. So flights are to adhere to these levels for safety reasons. Only because of "minimum fuel" or "emergency" etc. can the procedure be ignored.

[Old Smokey]

xfeed,

I assume that you mean B777-200 when you refer to 772. The default Step Climb in the standard B777 FMC (there are other customer options) is ICAO, i.e. 4000 feet. It is pilot selectable to other values as thousands of feet, typically 2000 feet in RVSM airspace, or 1000 feet in countries like Australia where ATC is quite flexible in allowing non-standard levels if traffic permits. The annoying bit is that 'odd-ball' steps such as 1600 feet are not selectable, very useful in China and some other countries where metric altitudes are still used with 'odd-ball' steps between standard levels.

The standard B777 FMC displays 3 significant levels.

First is the pure aerodynamic optimum level for the speed schedule chosen in still air. This increments upwards at about 100 feet every 7 to 8 minutes. Nice to have, BUT it displays still air values, which has little relevance in a windy world.

Second is the Max Alt, this considers Thrust limited level for the speed schedule chosen, Maximum level for buffet boundaries, or the maximum permitted level for the aircraft, whichever is the most limiting. Again, this increments upwards at 100 foot intervals.

Third, and most important for cost / fuel effective operations is the recommended level. This increments upwards at the set Step Interval as described earlier. If a good range of en-route winds and temperatures have been input above and below the nominal level, the recommended level will advise that for the best operation, and advise 'Step To' points in terms of Distance and Time. If a range of winds above and below the nominal level have not been input, it will simply recommend the standard level closest to the still air optimum level. When 'ducking under' Jetstream Headwinds, it's not uncommon to see something like Optimum Level 357, Maximum Level 374, and Recommended Level 290. Alternatively, when a positive wind gradient exists (increasing tailwind with increasing level), it's not uncommon to see something like Optimum Level 357, Maximum Level 374, and Recommended Level 370, that is, above Optimum Level, but at or below Maximum Level. (A lot of pilots will never see this phenomonom as it's common practice for many to simply insert wind at one level only).

A shortfall in the B777 system is also that whilst it allows several levels and winds to be input, only one 'reference' level with it's associated temperature is possible. It assumes standard lapse rates for the other levels, and this can be detrimental where the tropopause is well above standard (36089 ft), and much higher temperatures than actual are assumed at higher levels, delaying advice of the recommendation to step to a higher level.

If no significant wind gradient exists, the perfect means of achieving the desired Time / Fuel profile is to 'Cruise Climb' at the optimum level. Airspace is becoming too congested for this mostly, but as recently as last week I obtained 4000 foot block levels from Anchorage Centre (admittedly in a remote part of the world). Again, if no significant wind gradient exists, and block levels are not available, the pilot or the FMC must determine the most appropriate step point. For simplicity, this is typically done midway between the points where the present level, and the next projected level's performances are equal. The reality is that it should be approximately 2/3 of the way between. If cruise alone is considered, by all means step at the point where the 2 levels are equal, BUT a climb is required, and extensive analysis of the OVERALL fuel burn indicates waiting a little longer until the aircraft is lighter, and the incremental step climb fuel is therefore less, yielding the best OVERALL Time / Fuel for the sector.

That's my 2 cents worth.

Old Smokey

[xfeed]

Thanks everyone for the helpful replies. I hadn't taken wind forecasts into consideration so obviously that is an important factor. I was, indeed, referencing the B777-200 as that is one of my favourite planes and I appreciate your response Smokey.

Do airlines have a set Cost Index they set for a certain type of plane or is it standard across the fleet for that company? Can you change the CI in mid-flight?

Again, I appreciate the replies.

Cheers!

[Intruder]

Do airlines have a set Cost Index they set for a certain type of plane or is it standard across the fleet for that company? Can you change the CI in mid-flight?
Policy varies among companies, and changes with the price of fuel and the whim of management.

CI can be changed at any time.