Takeoff throttle engagement time

[skidoo]

So, my understanding that application of takeoff power on the turbo IO540 (T182T) should be gradual. I am told that is to prevent over boosting the turbo.

OK, so I tend to adhere to caution like this and I go ahead and apply power gradually. I don't know if it is a problem or not, but it seems that the power is so smooth and acceleration so great that by the time I get the throttle up to 32", I am already above 60 KIAS and ready to fly.

Is this typical, or should I be applying throttle a bit more quickly?

 

[Henning]

So, my understanding that application of takeoff power on the turbo IO540 (T182T) should be gradual. I am told that is to prevent over boosting the turbo.

OK, so I tend to adhere to caution like this and I go ahead and apply power gradually. I don't know if it is a problem or not, but it seems that the power is so smooth and acceleration so great that by the time I get the throttle up to 32", I am already above 60 KIAS and ready to fly.

Is this typical, or should I be applying throttle a bit more quickly?

As long as you have enough runway, you have no problems.

When you get into really high horsepowers and light weights, you often don't use full power. In the Bull Thrush (1435hp R-1820) empty, I'd stand on full right rudder and feed throttle at a rate that I could keep the plane straight (starting from a 45* angle right off the centerline) on the runway, by the time I had 1/4 throttle I'd be flying and still had to be careful with the throttle as I accelerated until I had enough speed that I wouldn't torque roll into the ground. When light, I'd rarely use over 1/2 throttle ever since it didn't add much for speed, but she'd haul 750 gallons when you asked her to.

 

[TangoWhiskey]

Here's what the POH for the T182T says on page 4-23 (I'm studying it now in prep for checkout):

TAKEOFF - POWER CHECK

It is important to check takeoff power early in the takeoff roll. Any sign of rough engine operation or sluggish engine acceleration is good cause for discontinuing the takeoff.

<skip paragraph about loose gravel areas>

On the first flight of the day when the throttle is advanced for takeoff, manifold pressure will normally exceed 32 in. Hg and fuel flow will exceed 24 GPH if the throttle is opened fully. On any takeoff, the manifold pressure should be monitored and the throttle set to provide 32 in. Hg; then, for maximum engine power, the mixture should be adjusted as required, during the initial takeoff roll, to 24 GPH fuel flow.

After full throttle is applied, adjust the throttle friction lock clockwise to prevent the throttle from creeping back from a maximum power position. Similar friction lock adjustments should be made as required in other flight conditions to maintain a fixed throttle setting.

I'm not sure why exceeding 32" Hg manifold pressure or 24 GPH fuel flow is more likely "on the first flight of the day"; can somebody explain that? The POH doesn't.

It sounds like full throttle, on a properly adjusted engine/fuel injector setup, should give you 32 / 24. It doesn't specify to apply throttle smoothly, but that's always good practice. You might consider applying smoothly, but faster, up to about 3/4 travel, let it settle out, then fine tune from there.

I found it interesting, too, that they advise adjusting the mixture on the roll. Isn't that my copilot's job? "Set max takeoff power".

 

[kgruber]

>>I found it interesting, too, that they advise adjusting the mixture on the roll. Isn't that my copilot's job? "Set max takeoff power".<<<

A LONG time ago it was. Now it's a good way of getting a bloody hand during an abort.

Neither Flight Safety or Simuflight allow the PNF to adjust the throttle/ power lever, without specific command from the PF. And, it is never a standard procedure.

 

[poadeleted20]

I'm not sure why exceeding 32" Hg manifold pressure or 24 GPH fuel flow is more likely "on the first flight of the day"; can somebody explain that? The POH doesn't.

Probably because the oil providing motive power to the waste gate is still cold, so the waste gate may be a little slow in dumping excess pressure. You can't exercise the waste gate the way you do the prop during run-up to get warm oil where it needs to be unless you do some full power throttle-ups, and that creates other issues.

 

[Dan Thomas]

Here's what the POH for the T182T says on page 4-23 (I'm studying it now in prep for checkout):



I'm not sure why exceeding 32" Hg manifold pressure or 24 GPH fuel flow is more likely "on the first flight of the day"; can somebody explain that? The POH doesn't.

It sounds like full throttle, on a properly adjusted engine/fuel injector setup, should give you 32 / 24. It doesn't specify to apply throttle smoothly, but that's always good practice. You might consider applying smoothly, but faster, up to about 3/4 travel, let it settle out, then fine tune from there.

I found it interesting, too, that they advise adjusting the mixture on the roll. Isn't that my copilot's job? "Set max takeoff power".

If the airplane has the hydraulically-operated wastegate, it runs on engine oil. When the oil is colder it takes longer to drain from the actuator, and the wastegate is held shut longer and the turbo can overboost the engine.

The hydraulic actuator has a spring to force the piston one way, and the oil pressure works it in the other direction. Engine oil pressure is plumbed to that cylinder, and oil exits from that same chamber to the controller, which either releases the oil back to the crankcase or restricts its escape. Releasing it allows the spring to move the piston, and restricting the oil makes it move things the other way. Cold oil flow sluggishly and the wastegate will react much more slowly.



Dan

 

[TangoWhiskey]

Probably because the oil providing motive power to the waste gate is still cold, so the waste gate may be a little slow in dumping excess pressure. You can't exercise the waste gate the way you do the prop during run-up to get warm oil where it needs to be unless you do some full power throttle-ups, and that creates other issues.

If the airplane has the hydraulically-operated wastegate, it runs on engine oil. When the oil is colder it takes longer to drain from the actuator, and the wastegate is held shut longer and the turbo can overboost the engine.

The hydraulic actuator has a spring to force the piston one way, and the oil pressure works it in the other direction. Engine oil pressure is plumbed to that cylinder, and oil exits from that same chamber to the controller, which either releases the oil back to the crankcase or restricts its escape. Releasing it allows the spring to move the piston, and restricting the oil makes it move things the other way. Cold oil flow sluggishly and the wastegate will react much more slowly.

Dan, I'll have to ask the mechanics or call Cessna to see if the waste gate is hydraulically activated--the POH doesn't say.

Ron, you're absolutely correct. A deeper dive into the POH, Section 7 Airplane & Stems Description, page 7-30, has the answer:

MOMENTARY OVERSHOOT OF MANIFOLD PRESSURE

Under some circumstances (such as rapid throttle movement, especially with cold oil), it is possible that the engine can be overboosted slightly above the maximum takeoff manifold pressure of 32 inches Hg. This would most likely be experienced during the takeoff roll or during a change to full throttle operation in flight. The induction air pressure relief valve will normally limit the overboost to 2 to 3 inches.

A slight overboost of 2 to 3 inches of manifold pressure is not considered detrimental to the engine as long as it is momentary. No corrective action is required when momentary overboost corrects itself and is followed by normal engine operation. However, if overboosting of this nature persists when oil temperature is normal or if the amount of overboost tends to exceed 3 inches or more, the throttle should be retarded to eliminate the overboost and the controller system, including the waste gate and relief valve, should be checked for necessary adjustment or replacement of components.

 

[Dan Thomas]

Dan, I'll have to ask the mechanics or call Cessna to see if the waste gate is hydraulically activated--the POH doesn't say.

If the POH talks about cold oil causing overboost, the wastegate is hydraulically actuated.

Dan

 

[TangoWhiskey]

If the POH talks about cold oil causing overboost, the wastegate is hydraulically actuated.

Dan

Ha. If I'd thought about that without a two glasses of Merlot mellowness, I'd probably have inferred that, too. ;-)

 

[Ted]

The T182 does have a hydraulically activated wastegate. Such things don't always have the best transient response (although I've still seen them do pretty darn well) but, when working, do steady state pretty well.

Ron's dead on regarding why overboost is more likely under cold operations.

What your definition of "slow" is is the real question. If you're like me, I'll frequently do a 3-4 second advance towards full power. If you feel that you're cutting your available runway a bit too short because of this, then simply stand on the brakes until you get up to 20-25" and then release. The 182s don't have any problems taking off in a short distance, though, so it probably doesn't make a big difference.