Thrust Reversers vs. Thrust Attenuators

[User546]

What on earth are the point of Thrust Attenuators?

I have considerable time in the Citation II, and currently fly a CJ2, and what in the world was Cessna thinking?

When you land in the II, once those reversers come out, there's no need to touch the brakes until your down past 60 kts (even then most of the time you've decelerated enough to coast off the runway without using brakes).

When you land in the CJ2, the Attenuators come out and you might as well just have your hand sticking out the window trying to stop the airplane that way. I will give the Attenuators the fact that they work good on the ground, by deploying and slowing the airplane down (requiring no braking action) when you bring the throttles back to the stops, but outside of that??

Now I understand it's two different principles working here, but what is a benefit of having Attenuators on an aircraft? No one I fly with is giving me a straight answer on it.

 

[Benny Hill]

Now I understand it's two different principles working here, but what is a benefit of having Attenuators on an aircraft?[/QUOTE]
The joke when the CJ first came out was that they were there to prevent birds from flying into the exaust end of the engine while in flight.

I think that they are only there for taxi assistance.
FYI-Cessna did not put them on the CJ3.

 

[User546]

That joke about the birds was hillarious! First good laugh I've had the past couple days.

Good to hear the CJ3 didn't good that pile of turd placed on it as well.

 

[Headwind]

The way I understand the CJ had to go from idle power to a given thrust in so many seconds time for the aircrafts certification. Don't remember the numbers now, but the Williams engine could not do it. To make it pass the test they had to set the idle speed so high it would be hard to taxi. The attenuators were installed for for this reason not to help you slow on landing. They are automatic on the ground when at idle speed. I guess they do help you slow and stop since you have zero thrust.

HEADWIND

 

[Big Duke Six]

Well, a side benefit might be the taxi assistance, but the certification issue was probably that the engines would not spool up in time in case of a go-around from idle power on final. Thus, the attenuators allow the engines to stay at a higher RPM while on final and still let the airplane come down. When go-around power is selected, the attenuators retract and POOF, you have thrust right off the bat while waiting for the engines to come up the rest of the way.


Another fine Cessna product, the T-37, had the same problem and the same solution.

 

[English]

Cessna put the attenuators on the CJ, CJ1 and CJ2 so you could wave at the people you left behind due to weight limitations. The CJ3 doesn't have the same weight limitations - thus, no need for the attenuators.

 

[Flying Illini]

stupid question...do the attenuators deploy in flight when the idle thrust is selected or are they controlled by a WOW switch or the like and only work on the ground?

 

[User546]

Not a stupid question...

They are controlled via squat switches on the ground, which disengage and lock the Attenuators after takeoff. There are three positions selectable on the switch: Auto, Test, and Stow.

If the Attenuators do come out in flight, it won't necesarrily mean catastrophic results, as some inflight thrust reverser deployments have caused. As long as you keep the airspeed from getting on the lower side, the CJ should handle well enough to get down safely.

 

[GravityHater]

Thus, the attenuators allow the engines to stay at a higher RPM while on final

Are there some a/c where the TA's deploy in flight??

 

[TonyC]

Are there some a/c where the TA's deploy in flight??

Review Big Duke Six's post - - the answer is yes, and the T-37 is an example. The purpose of the TAs in the T-37 is to allow a higher RPM without a higher thrust during approach. While clumsy, it allows the airplane to achieve acceleration from approach power setting to a go-around RPM in a shorter amount of time. Without the TAs in flight, the RPM for approach and landing would have to be significantly lower, and the spoolup time for go-around would be excessive.

 

[Big Duke Six]

Wow, when I heard "thrust attenuators" I assumed (there I go again) that they were the same as on the mighty Tweet. If the TA's on the CJ's don't come out during flight, then that seems like a lot of trouble for nearly nothing. Learn sumthin' new every day!

 

[joelo88]

heard that the CJ3 has no attenuators because the addition of FADEC has reduced the amount of idel thrust thru some magic. that's what they said in AOPA magazine at least

 

[Foxcow]

Are there some a/c where the TA's deploy in flight??

doesn't the C-17 have that capability?