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Purpose of ARM, THR HLD, HOLD modes

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I can't understand a part of boeing's autothrottle system. Apparently the Asiana pilots couldn't as well, considering the 777 crash was caused by this misunderstanding. The problem lies in the 777's a/t HOLD mode, which is followed by IDLE mode. The 737 has the similar ARM, THR HLD, and RETARD modes. In these modes the servos are disconnected from the engines and the pilot can manually set thrust. Why? What is the purpose of disconnecting the servos? Thank you.

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To allow pilot to manually adjust power (or cut it in case of RTO).

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Oh ok. But why is this mode also engaged when the aircraft is descending in FLCH or LVL CHG mode? Couldn't the a/t just stay at IDLE, RETARD? Thank you very much for the response!


To allow pilot to manually adjust power (or cut it in case of RTO).

Oh ok. But why is this mode also engaged when the aircraft is descending in FLCH or LVL CHG mode? Couldn't the a/t just stay at IDLE, RETARD?

 
Sorry i'm posting it again, I forgot to choose "quote"

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As I understood, a/t will go in HOLD mode during FLCH only if you try to overpower it. (it will basically say: You want control? Here you go!).

 

FLCH is very simple mode. Set idle power for descend or climb power for climb, and then pitch for speed. Now I do not know why would anyone want to change power setting while FLCH mode is active. The only thing coming to my mind is to adjust climb/descend profile, but there's V/S mode for that.

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In ARM mode during descent, the aircraft will retard power prior to it, and follow the vertical profile.  Sometimes you have to add or subtract power to maintain the speed bug, so the servos will disconnect. 

 

Let's say you have a tailwind on descent, and the wind suddenly starts dropping in speed. The IAS will drop below the pink speed bug. So, you would add a little power to maintain it.( unless you're already accelerating if the plane is on a steep descent.)

 

If you fall below the bug by a certain amount, the AT will "wake up" and throttle up to bring you at the right speed.

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As I understood, a/t will go in HOLD mode during FLCH only if you try to overpower it. (it will basically say: You want control? Here you go!).

 

FLCH is very simple mode. Set idle power for descend or climb power for climb, and then pitch for speed. Now I do not know why would anyone want to change power setting while FLCH mode is active. The only thing coming to my mind is to adjust climb/descend profile, but there's V/S mode for that.

Exactly! That's why i was confused with this mode.

 

In ARM mode during descent, the aircraft will retard power prior to it, and follow the vertical profile.  Sometimes you have to add or subtract power to maintain the speed bug, so the servos will disconnect. 

 

Let's say you have a tailwind on descent, and the wind suddenly starts dropping in speed. The IAS will drop below the pink speed bug. So, you would add a little power to maintain it.( unless you're already accelerating if the plane is on a steep descent.)

 

If you fall below the bug by a certain amount, the AT will "wake up" and throttle up to bring you at the right speed.

But don't FLCH and LVL CH control speed with pitch? If the IAS dropped, the aircraft would maintain idle thrust and pitch down, wouldn't it?

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Yes, but that's when you select it. I was talking about descending in Vnav.

 

Let's say you're on FLCH, but the plane is descending too quickly, you'll be a low on profile, below glide slope far away, etc.

 

So you increase power while on ARM, which makes airspeed increase, making the autopilot pitch up the plane resulting in lower Descent rate.

 

...Or you can just select vertical speed. lul.

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Yes, but that's when you select it. I was talking about descending in Vnav.

 

Let's say you're on FLCH, but the plane is descending too quickly, you'll be a low on profile, below glide slope far away, etc.

 

So you increase power while on ARM, which makes airspeed increase, making the autopilot pitch up the plane resulting in lower Descent rate.

 

...Or you can just select vertical speed. lul.

Ah, ok! Makes more sense now. But you could also just increase the speed in the MCP right? haha. That and or selecting v/s seem like better options. Thank you very much guys.

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