What is cruse climb techniques?

[ericliu]

The definition of cruse climb technique is "A climb technique employed by aircraft, usually at a constant power setting, resulting in an increase of altitude as the aircraft weight decreases."
Does this mean what the pilot only does is to keep the constant power, and the aircraft would climb by itself due to lose of weight?
In what conditions this technique would be applied?

[PACIFIC BARON]

Cruise Climb assumes the ability to maintain optimum altitude at all times which for 99.9% of all commercial operations is not possible due to ATC requirements. In theory the ability to fly along at your optimum altitude for the entire cruise given other parameters equal, e.g. temperature, wind etc , then one would achieve the minimum fuel burn for the same conditions, cost index or Mach number. However, in reality, in most circumstances one can only approximate cruise climb which we call step climb. Be it changing altitude in small increments, e.g. 1000ft or 2000ft at defined intervals will approximate a cruise climb at very little penalty & keep the aircraft in compliance with ATC requirements.

It is possible to cruise climb if an aircraft is given a block altitude clearance however, most airlines to my knowledge do not approve this technique on safety grounds versus little fuel savings.

[Pedalz]

Eric,

To answer your second question, in what conditions would this type of climb be applied, a prime example would be concorde, which continously climbed (aslong as conditions such as winds aloft and even solar radiation did not hamper this) throughout cruise. This was acceptable to both BA and Air France due to nil other traffic operating in this altitude block and helped to keep the fuel costs down for the bean counters and greenies :suspect:.

[conflict alert]

Cruise climb or (cruse!!! climb) from an ATC point of view in NZ means go forward fast at the cost of climbing. Generally used from an ATC point to keep a climbing aircraft ahead of another aircraft or from a pilots point (so I gather although they rarely ask for it and just do it) running behind schedule!! and their climb performance is not that expected for the aircraft type, which, at times, we base seperations on (hence why it is subject to a request and clearance for said procedure!). Also used (on our sector alot) is cruise descent by ATC which is generally used to push an aircraft down from a specific altitude with the expectation that the aircraft will not descend at the vertical speed normally expected for that aircraft type - used for speeding the aircraft up to maintain or increase distance from a following aircraft and allowing the pax and their coffees to be finished in style.

Does that make sense??

[A Comfy Chair]

Conflict alert - I've never seen a proper cruise climb used for that! In a cruise climb, forward speed does not change. Basically the aircraft is allowed to slowly climb to maintain optimum altitude.

For what its worth, our airline prohibits them.

[DFC]

Lift opposes weight and drag opposes thrust.

Generating lift causes drag.

As weight decreases (fuel is burnt), the amount of lift required is reduced.

Therefore the drag is reduced.

If nothing is done, the aircraft will speed up.

Very broadly speaking, with jet engines the higher one goes then the more fuel efficient the engine.

There will be a certain altitude where with all the above forces, the speed is at the most efficient speed for the aircraft / conditions and the engine is operating at it's most efficient setting i.e you are getting the best from the aircraft.

As fuel is burned weight decreases and as I said above the drag decreases meaning that the amount of thrust required to maintain the ideal speed is less. However, if you throttle back then the engine is no longer operating at the most efficient setting.

The ideal answer is to maintain the same speed and the same ideal power setting but using the now excess thrust to (slowly) climb the aircraft.

Since fuel is constantly being burnt then weight is constantly decreasing so there is a continuous excess of thrust required which is used to climb the aircraft.

Thus the perfect way to operate a jet aircraft is the cruise climb technique as described.

In practice most flights can't do that due to ATS restrictions and will compromise by using step climb i.e. initial climb to perfect level and cruise at that level for longer than it efficient until the weight is low enough to climb to the next level where the process is repeated.

There are 100 other factors involved but the above is a brief rounded explanation of the "cruise climb technique".

Regards,

DFC

[conflict alert]

when requesting a pilot to make a 'cruise climb' we see a significant increase in the ground speed with a much lower rate of climb. That would be the expectation over here. I guess its just what each country is used to in 'their' procedures. Its used mainly on turboprops and the likes not the jets.

Having re-read the thread starter it does say cruise (cruse) climb technique which is probably a bit different to what I'm talking about (I think!)

[conflict alert]

And it would appear that DFC's reply confirms this :) ie we are on different paths

[Bergerie1]

Cruise climb is without doubt the most fuel efficient cruise technique. Concorde used it (there were no other aircraft at 50,000ft+ and it was the only way to carry an economic payload), and Boeing considered it when they were developing the Sonic Cruiser which, had it been built, was designed to fly above the cruise altitudes of current commercial aircraft. But cruise climb can only be done when it is possible to block off bands of altitude, which in turn, is only possible when there are no other aircraft which could cause conflicts.

Current FMS equipment has what is called a Cruise Climb Mode but it is not a true cruise climb, it is only a means of climbing between the existing and a new cruise altitude. Climb thrust is used and speed is controlled by elevator.

An ideal cruise climb would need a new FMS mode, not difficult to do but not done because there is no demand for it from the operators since ATC procedures cannot support it without limiting airspace capacity by blocking off altitude bands. However, the cruise climb mode would need to command the optimum thrust (which would would change with pressure altitude and temperature), would control speed on elevator (the speed being varied to be ideal for the altitude, temperature and head/tailwind component - faster for headwind/slower for tailwind), and I guess one would then use the best range speed modified also for cost index. I am not a performance expert but I have talked with those in the know and it is not difficult to do.

Studies have been done on the possible benefits and I have seen estimates of between 5% for business jets (which I think is highly over optimistic) to less than 0.5%. Airbus recently did some work on A330 aircraft which showed that for a 4000nm flight between Europe and the Caribbean, at M0.82, at max payload, in still air, and with a continuous cruise climb to FL 390 the theoretical savings were 0.67% when compared with step climbs of 1000ft, and 1.2% when compared with step climbs of 2000ft.

Perhaps if ASAS Self Separation becomes a reality this could open the way for cruise climb techniques in low density oceanic airspace. In these days of expensive fuel and the need to reduce CO2 it would be worth trying.