Question for Saab 340 Drivers

[Typhoon1244]

My brother has tried to explain this to me in the past, and I'm sorry to say that each time my eyes glaxed over and I nodded and said, "Oh. Okay. Right." But the fact is I still don't get it.

In fifty words or less, what the heck is a C-TOT?

 

[Otto_Pilot]

It's been a while...


CTOT - Constant Torque on Takeoff

But as I remember (in 50 words or less) CTOT was a poor-man's autothrottle. You could set the "dial-a-torque" prior to takeoff, advance the power to within 20% or so and hit the CTOT switches and the ECU would advance the power (fuel flow changed, the power levers didnt move) to the torque you selected. It would hold it at that torque throughout takeoff and initial climb. You didnt have to worry about ram-rise etc.

After takeoff you would dial it down and turn it off and you were back to a regular airplane again.

I often wondered why more airplanes didnt have a similar system for setting exact thrust numbers on takeoff. Seems like that would have been a nice addition for CRJ operators who elected not to have the autothrottles installed.

 

[acarpe3448]

OTTo-Pilot gives a pretty accurate discription. If I were to add anything it would be,

The CTOT circuit which is located in the engines ECU signals the torque motor to meter fuel beyond the power lever position to obtain a preselected Torque value set on the CTOT Panel. Advancing the power levers beyond the selected torque value will override the CTOT and provide the torque selected by power lever position, ie. CTOT set to 90% power levers pushed up to 108%, 108% Tq will be provided. To enable the system, condition levers must be in the min max range power levers above the 64 degree switch and the CTOT switches placed to the on position. The CTOT switch also arms the APR system on the B-Model.

 

[jjbiv]

Anyone care to do a similar description of the APR system?

Thanks,

joe

 

[acarpe3448]

APR=Auto Power Reserve.

Should an engine fail on takeoff the power turbine sensor will see the drop in RPM. The sensor will give this information to the ECU which in turn sees what is set in the CTOT and what the actual rpm is. When the ECU sees the difference between CTOT setting and actual power turbine speed, the ECU will tell the HMU to add more fuel via the torque motor. The torque motor opens more fuel is added and the good engine will provide an additional 7% of torque. When the power turbine sensor sees that it is spining at this new higher speed the ECU and HMU work together via the Torque motor to regulate the flow of additional fuel to maintain this power setting. This system is only on the B-model. I have got to leave home for a while now, anyone care to take it to the next level and explain how the Auto Coarsen system fits into the CTOT-APR equation. Remember to specify the conditions to arm as well as to activate the system for both A and B models. Gosh I hated this system so much during my Type Oral! This along with the overspeed protections (Crossair Fix etc. ) make for a very boaring night of study!

 

[DoinTime]

I often wondered why more airplanes didnt have a similar system for setting exact thrust numbers on takeoff. Seems like that would have been a nice addition for CRJ operators who elected not to have the autothrottles installed.

I don't believe that autothrottles were ever an option on the CRJ. Some Challengers were available with the autothrottles but they have a different avionics package.

 

[saabcaptain]

What makes this a "poor man's autothrottle" is the system has no way to decrease fuel to the engine so it is necessary to set the power levers below the desired setting and allow the CTOT system to add fuel to exactly set a desired torque amount. As ram rise occurs during the takeoff roll the CTOT system adds less and less fuel to keep the torque steady. If you overshoot the torque setting with the power levers or get too close so that ram rise takes you above the desired torque setting even with CTOT supplying zero extra fuel the system as I said can't subtract fuel so you will end up with excess torque.

It works pretty well and prevents the FO from having to tweak the power levers to match a desired settings.

 

[FL000]

Anyone care to do a similar description of the APR system?

Thanks,

joe

Lose one engine on takeoff, other one automatically rises 2% N1. Any questions?

 

[saabcaptain]

Actually in the Saab if you lose one the other side gains 7% torque.

 

[FL000]

Sorry, I was being aero-centric and forgot the thread topic.