Question about the THR switch on the 757 MCP

[Chiefttp]

I remember hearing that there are a few inputs that activate the THR/N1/EPR switch on the 757 /767 MCP. In order for the switch to engage a few parameters needed to be met. One being, a takeoff thrust setting must be set. Does anyone know what other parameters are required in order for this switch to engage?

[deltahotel]

A/T arm switch on!

[deltahotel]

Also ac on ground, flaps not fully retracted

[Chiefttp]

Deltahotel,

your response brings up a good point, if the flaps are set at 30, which is not a takeoff flap setting, will that switch engage? The genesis of my question stems from an event at my airline where a crew mistakenly set CLB 2 in the Thrust Management panel. When they pressed the THR switch at the start of their takeoff roll it wouldn’t engage. They manually set the thrust levers to the Caret (which was CLB 2) and almost had a tail scrape. I have flown other aircraft that have a takeoff light which will only illuminate when certain takeoff parameters and settings are met. The 757/767 has no such system, but I explained to my F/O that if the THR switch doesn’t engage when pressed at the beginning of your takeoff roll, there may be a good reason why. I’m just trying to tap into the great minds at PPRUNe to see if anyone knows more about this system. The manuals leave a lot to be desired.

[deltahotel]

Wording straight from B12 and those three are the only quoted conditions (apart from switching the system on!). F30 should get picked up by Config warning.

rgds

[deltahotel]

I suppose a light could be nice, but failing to engage on selection achieves the same aim (or should) of saying ‘I’m not ready to go- maybe you need to check something’.

[Chiefttp]

[QUOTE=deltahotel;11531151]I suppose a light could be nice, but failing to engage on selection achieves the same aim (or should) of saying ‘I’m not ready to go- maybe you need to check something’.[/QUOTE
.
I agree, but I wish I had specific conditions that activate that switch. On a prior Jet (Lockheed) we had 8 conditions that had to be met before we got a “Takeoff Light” . The Boeings have no such system, but the THR switch acts in a similar manner. I just wish I had more definitive info on the inputs that trigger that switch,
Thank You for your response.

[oceancrosser]

Caveat: Way too many hrs on 757/767 (15K)
https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/689x751/img_0055_88207b545bdf699ba54dab23c9863d979bbafdec.jpeg
But looking at this picture (identical one for a/c with EPR switch), it seems that only the A/T arm switch ON switch is required. CONFIG WARNING takes care of Flaps being in t/o range. I have had a situation where we pushed the button and nothing happended. Took a good look around, no warnings, cautions or Status msg. A/T arm switch Off and off we went with a manual throttle. Busy day, but if the TMSP is ok, that helps.

[deltahotel]

Had another look at the B12s. 767 gives the three requirements- flaps, TO setting, ac on ground so that’s all Mr Boeing wants/needs. 757 doesn’t mention any of these so I can only assume it’s the same but I’m not about to try ithem next time I fly it!

[Chiefttp]

So nothing about a Takeoff rated thrust setting being selected? I can guarantee you, if you set CLB , or CLB 1 or 2, instead of a takeoff thrust setting, the THR switch iwill not engage. This was a teaching point our instructors would do in the sim to reinforce the habit of confirming the Thrust setting selected in the TMP.

[deltahotel]

I think I said TO setting. Here you go cut n paste

The Aircraft is on the ground


Takeoff thrust limit (TO) shows on the EICAS display


The Falps are not fully retracted

[Chiefttp]

Thanks!

[rudestuff]

On the 767, THR pressed in cruise will definitely cause the thrust levers to advance and have you reaching for the disconnect switch. Useful for overcoming the 125-second logic in a climb but not a lot else.