PDA

View Full Version : Derated Takeoff on Turboprops


The Stooge
15th Oct 2004, 01:40
I know that there has been quiet a bit on this subject however I cant find too much relating to a TP. So could some one please help me with this delemer, which is that out company uses derated takeoffs on a GE CT7 and then when we set climb power we are using much higher power levels than what was used for takeoff. IE we might use 84% for Takeoff and then when we set climb power after turning the bleeds back on the power chart will say that we can have 100%. We have no logic in keeping the power set at T/O value until we intercept it on the climb chart. Surely pushing the power leavers forward gives the engine more fuel thus means hotter itt's. Thus meaning defeating the purpose of the derated takeoff. So if anyone can shed some more light on this subject it would be greatly appreciated.

oldebloke
15th Oct 2004, 08:40
There's some mention of 'reduced power'takeoff's in the Metro3 manual..(95%limit i believe)
When 'reduced' takeoff's were first developed there was o general understanding that it wouldn't be set 'less than climb power..To make the cert' requirements??(the a/craft had to meet the climb gradients)..Keeping in mind that the first thing one does,with a fire failure is to select 'power'to the max...
Cheers...:ok:

Old Smokey
15th Oct 2004, 12:30
These days Reduced Thrust / Derated Thrust is a pretty exact science, but the problem that you mention was fairly commonplace in the early days of reduced thrust takeoffs. It was pretty much left to the operator to use appropriate procedures to avoid negating the advantages of the reduced thrust / power takeoff that you allude to.

The first aircraft that I operated using the reduced thrust principal was to either -

(1) Leave the Thrust / Power where it is if an increase would be involved in setting Climb Thrust / Power, OR

(2) Set Cruise Thrust / Power as the initial setting for climb after Takeoff. (I know of no case where reduced Takeoff Thrust / Power was less than that for cruise (but there's always a first time).

Option (2) was finally settled upon as the best solution, with later increase to normal Climb settings at higher levels when R.O.C. had fallen to a nominal value (1000 fpm for the aircraft I speak of).

What I've said here related to jet aircraft, but the essential principals are the same.

Hope that this helps,

Smokey

Cardinal
17th Oct 2004, 05:33
Stooge, I feel your pain. We fly the B1900D/PT6 and do a similarly illogical thing. We carefully look up the torque value for "Reduced Takeoff Power - Bleeds Open" which ends up being 3200 ft/lbs or so. Then at 400 feet we call for "Climb Power," adjust the prop RPM, and invariably nudge the power levers up to Max Continuous Torque of 3750 ft/lbs. Reduced torque/itt for 30 seconds on the roll, then those same values are at the edge of the yellow arc for the next 15 minutes of climb? Makes no sense to me. For that matter, our "book" cruise power settings that we are repeatedly urged to adhere to actually equal max continuous torque all the way up to 13000ft or so.

I suspect this is a smoke and mirrors thing on the company's behalf, either to make a power-by-the-hour deal sweeter or establish a more generous inspection schedule with the FAA.

To further confuse the issue, our full power numbers are bleeds closed, yet our reduced numbers are for bleeds open. If ITTs are truly a concern, why not close the bleeds?

To throw one more wrench into the works, what prevents turboprops from using the relatively simple assumed temperature method instead of a whole different table of power settings and Vspeeds? On a cold day we use the first 1/8th of a 16000ft runway, seems like such a waste.

But I shouldn't expect much. After all, our manuals also indicate that while passenger emergency O2 consumption increases with cabin altitude, crewmember O2 consumption actually decreases. Pilots require only half the flow at FL250 that they would at FL180, such that by FL400, the crew wouldn't need any oxygen at all!! In the event of a depressurization just climb, right? Figure that one out.

SmolaTheMedevacGuy
17th Oct 2004, 15:11
Doesn't the prop RPM have to do with the issue? During the climb sequence, the reduction of Np will give an increase in the torque values, as:

TQ=Power/Np

So if the Np decrease more than the power, the TQ values must rise (for ATR 42-500 TQ typically rising from 90% to ~109% after selecting CLB, while the actual power decreases) Just guessing...

On the oter hand, on ATR 42-500, when changing from CLB to CRZ, the torqes (and the fuel flow and ITT) increase, with Np being constant at 82%. Seems that CRZ pwr is higher than CLB. Nobody (at least in my company) is able to explain this phenomenon...

Greetings
Smola