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zonoma
14th Jun 2019, 22:13
Hi all, ATCO here so please be gentle. My employer has recently alerted us that several B787 operators will be flying their aircraft in a different mode so we can expect them to climb at a lower rate than usual. We get that, but today one particular aircraft operating as such was climbing at 500ft/min but pushing out 348kts IAS. We are told that the new profiles are to "protect the engines" and the question was asked - if say the new model sets aircraft thrust at 78%, why climb at 500ft/min with 348kts IAS, why not climb at 1000ft/min at 315kts (or whatever the slower speed would be). We think it is simply "set thrust level ie 78%, trade off speed for climb rate" and the engines are still protected. Can someone please try and educate me in simple terms what we as controllers are missing??! Thanks

ScepticalOptomist
14th Jun 2019, 23:21
There are a few trains of thought but one is - maintain close to whatever speed you were cruising at for the climb - climb rate will be slower but thrust wonít go to maximum for the climb, especially for a gentle 500í/min. As a bonus, itís also quieter and wonít disturb the passengers who may be trying to sleep.

You donít really want to have to accelerate once level as at altitude it can take a long time and a lot more thrust.

zonoma
14th Jun 2019, 23:30
Thanks for the answer. To put more input then, it's all flights day or night, and this is from the ground upwards. The aircraft concerned today spoke to me between FL165 & FL265 on what I'd consider a medium length sector (around 9 hour flight time).

averow
14th Jun 2019, 23:32
Quick question: could there be some small gains for the operator in terms of fuel economy and/or wear and tear on the engines ? It might end up being a complex enough scenario for the operators to do some computer modeling about the various tradeoffs involved.

ScepticalOptomist
14th Jun 2019, 23:34
In that case, Iím not sure what their rationale was - maybe, if they were being step climbed or only cleared a few thousand feet higher each time, they were trying to avoid levelling off between clearances?

CaptainProp
14th Jun 2019, 23:53
Constant thrust, slowly decreasing with climb, to keep engine protected from large temperature changes that comes with constant changes in thrust during normal climbs with level off segments?

CP

Sidestick_n_Rudder
15th Jun 2019, 07:28
As far as I know slower 787 climbs are due to RR engine issues and RR recommendation to use CLB2 whenever possible to prevent engine damage. Also, some operators modify climb derate washout to keep derated climb thrust until TOC. This generally results in slow-is climb rates especially in the mid20ís to low30s.

However, climbing at 348kt doesnít make sense, Iíd venture it was someoneís not-so-bright idea rather than airline SOP.

zonoma
15th Jun 2019, 09:12
Thanks all, it is a RR protection model & your replies make sense.

Tricia Takanawa
15th Jun 2019, 09:47
Whilst related, they are different settings for the "computer". The speed would be related to cost index, which the airline specifies for each flight. That speed would indicate that they were most likely trying to make up time, or someone went rogue and was flying faster than plan.
The climb thrust is a separate setting, and is usually dictated by company SOP and default settings.
The aeroplane will only wash off the reduced climb thrust very gradually if at all. So by flying away from the best rate of climb speed (which is most flights by differing amounts), these climb rates are all that you are left with, unless someone manually adjust the climb thrust in the FMS.