AUTO THROTTLES

DE727UPS

Well-Known Member
Well....I'm no expert yet....but I have to do 757 training soon.

Basically, you can select what you want the throttles to do for various modes of flight.

On takeoff, you push the EPR button and the throttles go to the selected takeoff EPR setting, then climb thrust at 400 AGL.

For climb or descent, you can select Flight Level Change, and the throttle will give you climb thrust or idle for descent.

If you want to hold a certain speed, you can select speed mode and the throttles will automatically give you the thrust to hold that speed.

I didn't read the chapter on VNAV yet...but supposedly the plane just fly's itself around the way you programed the FMC.
 

A300Capt

Freight Dawg
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Can someone axplain in simple terms how do autothrottles work?


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Simply put, auto-throttles are to an airplane what cruise control is for a car!


The Auto-throttle system will manipulate the throttles to maintain a programmed FMC speed and/or thrust setting or will maintain whatever speed the pilot manually dials into the mode control panel (dashboard panel).

There's a lot more to understanding the auto-throttle system than that but that's it in a nut shell.
 

A300Capt

Freight Dawg
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On takeoff, you push the EPR button and the throttles go to the selected takeoff EPR setting, then climb thrust at 400 AGL.


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Actually, climb thrust at 1000' when you call "Vnav, flaps 5". The auto-throttles automatically go to climb thrust when Vnav is selected. Some airlines have to select "climb" on the TMCP after selecting Vnav but ours are wired together to happen simultaneously.

You'll soon know all the secrets of the "Atari Ferrari".. young Jedi.
Enjoy it, Don! It was the best airplane I ever flew and it'll water your eyes with how smooth the automation is and how well it takes care of you on those long nights.
 

IrishSheepdog

Sitting in the median
A300Capt, I know you flew the 757/767. When did you use Climb 1/2 modes? Was there a specific SOP regarding using the less-powered climb modes?

(Aside: It was very tricky trying to hand fly with autothrottled in the 737-300 sim! Every altitude bust was exaggerated since the autothrottles would add or remove power as you climbed or descended, respectively.)
 

A300Capt

Freight Dawg
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A300Capt, I know you flew the 757/767. When did you use Climb 1/2 modes? Was there a specific SOP regarding using the less-powered climb modes?

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Anytime you selected a reduced t/o power setting (derate 1 or 2) the default for climb power would be climb 1 or 2 (ex: derate 1 t/o would automatically revert to climb 1 etc). We could also do an "assume" (or "flex" for the Airbus fans) temp reduction off the derate setting. Confused?
Yea, it takes some getting use to but becomes clear after some use. The use of reduced t/o and climb power settings is SOP except during inclement wx conditions or MEL considerations.



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(Aside: It was very tricky trying to hand fly with autothrottled in the 737-300 sim! Every altitude bust was exaggerated since the autothrottles would add or remove power as you climbed or descended, respectively.)


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Funny thing you mentioned that. 727 guys/gal have a heck of a time initially when flying the B75/76 or even the 737. Reason being, the 727's thrust is inline with it's CG. You feel a push when power is applied but there's relatively little pitch change associated with it. Same with power reduction.

With the wing mounted engines on the B75/76/73 etc, the thrust line is below the CG. We know that everything pivots around the CG. When power is applied and with the thrustline being below the CG, the nose has a tendency to pitch up relative to the amount of thrust being applied. The opposite is true when power is reduced.

With me so far? Good.


Now, introduce autothrottles into the mix. Let's say you're hand flying an ILS with the autothrottles engaged to "speed" mode. If you raise the nose on any airplane with a constant power setting the airspeed will gradually bleed off. If you let go of the yoke the nose will drop to try and regain the speed it was trimmed for (nothing new..basic stuff here). Now, with autothrottles, the problem is that when you raise the nose slightly the speed bleeds off and the autothrottles compensate with more power. The more power now increases the nose pitch attitude..reducing speed again and...increasing more power by the autothrottles etc, etc, etc. Can you see the cycle? The reverse is true if you push the nose over. Speed increases so the autothrottles reduce power which in turn pitches the nose over more...increasing speed....more power reduction etc. It's a dog chasing it's tail scenario. The new crewmember is chasing the autothrottles while the autothrottles are chasing the pilot.

It's recommended that if you're hand flying to turn the autothrottles off. Experienced crewmembers, familiar with the idiosyncrasy of the autothrottle system still prefer to hand fly with them engaged.

I was a B75/76 sim instructor for many years and most people new to glass cockpits and autothrottles fought with them while hand flying an approach initially. I made it a point to explain the whole potential negative stability autothrottle thing to them prior to the first sim period and most appreciated that advice.
 

IrishSheepdog

Sitting in the median
Nice! That is some interesting stuff. Since our engines are wing mounted basically straight down the wing (actually slightly), we don't have these problems all too much. We also don't have autothrottles (although we do have a automatic takeoff power system).

I understand what you mean about the flex power. We do flex power takeoffs as much as possible, to comply with the GE lease agreements. However such things as a contaminated runway, or deice, will cause us to do a rated power takeoff. I guess it's similar to your derated takeoffs, except we have straight climb power settings rather than derated. We can use 76% torque for takeoff, but then use 100% torque for climb.
 

dakrzyhwn80

New Member
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Can someone axplain in simple terms how do autothrottles work?


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Simply Put: There is a Desired input and an Actual Input. A desired input is a pilot selected input. An Actual input is the current position of a control such as flaps, ailerons, elevators, spoilers, etc....

Desired inputs are "Dialed In" on a computer by the pilot. The input is then sent to a servo, which controls what you want controlled. In this case it will be the fuel flow.

Once sent to the servo, the controls adjust to the pilots Desired input.

Feedback from the control (through an ARINC bus) transmits a signal of the curent control position back to the servo.

The survo will still actuate the contol surface until the Actual input zero out, or is equal to, the Desired input.


Ex. Pilot selects 20 degrees of flaps as a Desired input. This signal goes to the Flap Servo. Flap Servo is actuated to move the flaps to 20 degrees. Flap position is fed back to match the Desired input.

Such as...Desired input 20 Degrees: Actual Input 2 degrees....4 degrees...8 degrees....10 degrees...20 degrees.

When the Actual input is zeroed out - the servo stops


Hope this makes sense
 

DE727UPS

Well-Known Member
A300Capt...something finally dawned on me after about 15 years. I made it as far as the sim check at World Airways in the late 80's. While doing the ILS flight director with the autothrottles on I got into into the speed chasing scenario you speak of....I thought it was just me all those years. Anyhow, I didn't get hired, which, in the end, might have been just as well....

I'll look out for this in the sim. My understanding is they want you to use the automation all the time so we'll see what happens.

I've read the systems manual 1.5 times and done the CBT once already and I still haven't officially recieved my books yet...just borrowed stuff from friends.
 

A300Capt

Freight Dawg
Don,

Don't be afraid to grab ahold of those autothrottles and "augment" their movement. Too many times I've seen folks sit there and watch the autothrottles do things they knew wasn't right but didn't know how to intervene. The autothrottles won't disconnect even if you slam'em around from firewall to idle unless you pickle the thumb disconnect switch and they won't fight you, ah...well..like the Scarebus autothrottles do.


If you feel the power coming up under your hand simply because you momentarily raised the nose (but didn't mean to), simply grab the throttles and pull them back a little or stop their movement. In a word...be pro-active and not just a passive observer with the autothrottles. When everything's under control, release your hand pressure and let the autothrottles do there thing. Smoothness is key to flying an ILS with autothrottles engaged. Minor pitch changes=minor autothrottle movement.

Here's another tip:

The B75/76 have BIG windows. That means that the rwy will appear to be about in the middle of the windscreen on a normal visual/ILS approach instead of near the bottom like in the B727. The first time you see this I'll guarantee you'll instinctively try to raise the nose to put the rwy on the point of the windscreen you're use to seeing the last 10 or so years. The impulse to do this becomes greater the closer to the rwy you get and then whole thing about chasing the autothrottles comes into play. Everybody goes high initially and/or begins to flare too high until they get the sight picture down. The big forward windows create this illusion. Remember that when you first get into the sim training.

My advice and I became horse more than once saying it to students over and over: Fly the flight director!! It's 10 times better than what you had on the B727 and it'll be your best friend hand flying single engine ILS's to 100' in the sim.

Have fun!

Bill
 

A300Capt

Freight Dawg
FlyChicaga,

Yea, sounds like your procedures are inline with what we do.

I try to explain that using the Derate 1 or 2 button on the TMCP is like physically removing the PW2040's we have on our 75's (rated around 41,000lbs thrust)and installing PW2037's engines (about 37,000lbs of thrust) on the wing. We can then "assume" or "flex" temp off that new rated power.

As I'm sure you know we are limited to how much we can reduce t/o power and it can never be less than climb power.
 
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