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G/A Thrust limit

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Author Post
Member
Registered: May 2011
Posts: 64
Location: Dubai
Hello Hardy,

I have a scenario where the thrust limit should change to G/A, but it doesn't. Not sure if I am missing something but here it is...

Say you do a take off and right after take off you need to return for an immediate landing for what ever reason, uncontrollable engine fire or smoke in the flight deck.

So you level of at 1500' keep flaps 10 with flaps 10 speed or even clean up and ask for vectors back for an approach.

The thrust limit will still be CLB as the active VNAV phase is the climb phase.

Now you change the destination back to the departure airport in the FMC select the approach, extend the centre line etc etc...

Now what I have noticed is the thrust limit never changes to G/A on that return approach. Not when you select flaps again if you cleaned up or at latest G/S intercept.

Maybe it has something to do with the fact that you never reached you cruise altitude so never went into the DES phase on the FMC.

Any thoughts?

Regards,
Chris
Moderator
Registered: May 2009
Posts: 5140
Hi Chris,

yes, when you are in ACT CLB the flap-out condition won't auto-activate the G/A limit. If it would, you would get G/A after takeoff.

So in your emergency return you need to select G/A manually -- if you are able to go around at all.

Or, if you have the time, set your CRZ ALT to 1500.


Cheers,

|-|ardy
Member
Registered: May 2011
Posts: 64
Location: Dubai
Hi Hardy,

Thanks for the reply.

I would've thought the 744 would be similar to the 777 in this regard.

The thrust limit is locked in G/A in the 777 when the G/S is captured or the flaps are in the landing position.

Why doesn't the limit change to G/A in PSX when the G/S is captured as it says in the PSX manual? ( In the scenario I mentioned)

Thanks
Cheers
Chris
Moderator
Registered: May 2009
Posts: 5140
As far as I know, the 744 logic doesn't check whether landing flaps are set; it just checks whether the flaps are out, i.e. the G/A limit already activates when you set flaps 1, not only at flaps 25.

In PSX, both the G/S check and flap check are bypassed as long as the FMC is in ACT CLB.

Do you think the G/S check should never be bypassed?

(Another quick solution to get into the FMC DES mode is to press the DES NOW key on the DES page.)


Cheers,

|-|ardy
Member
Registered: May 2011
Posts: 64
Location: Dubai
I would think the G/S & landing flap (777) check is like the "last line of defence" if you can call it that to ensure you have the G/A limit set.

So if you do a quick return or even just a return not having reached your cruising altitude you know you will have G/A thrust available.

The landing checklist does not ask you to ensure that G\A limit is set so I gues the system was designed so that no matter what you will have the G/A limit available by final approach.

I can only speak for the 777, any real 744 drivers that can shed some light maybe?

Cheers
Chris
Member
Registered: May 2010
Posts: 843
Hardy wrote
As far as I know, the 744 logic doesn't check whether landing flaps are set; it just checks whether the flaps are out, i.e. the G/A limit already activates when you set flaps 1, not only at flaps 25.


Do your references say "flaps not 0" or "flaps extended"?

Perhaps the logic is smart and knows the initial flap setting and the new flap setting. If the flaps are commanded in the extended direction, the mode changes to GA(?)

Rgds
JHW
Member
Registered: May 2010
Posts: 843
Christo van der Loo wrote
The thrust limit is locked in G/A in the 777 when the G/S is captured or the flaps are in the landing position.


I'm having trouble understanding the relationship between A/P G/A pre-conditions and GA thrust limit (if any) on both aircraft

The 777 Maintenance Manual says:

"The go-around (GA) mode engages when all these conditions are true:

Autothrottle arm switches on
Airplane in the air
Glideslope active (engaged) or flaps not zero
Thrust limit not takeoff (TO)
Push either TO/GA lever.

Operation
The GA mode arms in approach when the flaps are down or glideslope is active. GA showsas the thrust limit mode on the EICAS display.
"

"Thrust Limits

The thrust limit calculation for A/T modes uses these inputs:
CDU thrust limit mode request
MCP A/T mode request
Air/Ground
Flaps not up or glideslope capture
Ambient conditions (ALT, TEMP and pressure).
"

This seems to disagree with your earlier statement that GA appears with the flaps in the landing position.

Rgds
JHW
Member
Registered: May 2010
Posts: 843
In this 777 video, the G/A thrust limit seems set before G/S capture or landing flaps

https://www.youtube.com/watch?v=bCiZNhOb71w

FMC entry?

Rgds
JHW

(EDIT) Not sure now. CRZ and G/A look similar at low res. Unfortunately, the F/O's sleeve covers the display when the flaps are selected out of up.
« Last edit by John H Watson on Sat, 13 Jun 2015 23:52:47 +0000. »
Member
Registered: Aug 2014
Posts: 24
Member
Registered: May 2011
Posts: 64
Location: Dubai
Hi guys!

When I mentioned the G/S and landing flap position I was talking about a quick return or return if you didn't reach you cruise altitude. And you didn't manually select the G/A limit or DES in the FMC

In normal operation on the 777 the mode changes to G/A as soon as the flaps are out of up as well. Plus a lot of other parameters as well. I know that, I only mentiontioned the landing flap and G/S because if you would do the normal CLB CRZ DES phases you will have G/A set long before you reach the G/S or when landing flap is selected i.e when you extend the flap.

So what will happen with a quick return to a non-precision approach. Then there is no G/S check to reference to because there is no G/S. So I guess that is why the landing flap is there.

This is a quote from the 777 FCOM

"The reference thrust limit is locked in GA when flaps are in landing position or glideslope is captured." I didn't say set. So this means no matter what you do you limit will be G/A by the time you are landing.

I have no issues with the normal proceudres. I am just wondering about a "non normal" situation if you didn't go through ACT CLB CRZ DES phases. Do you then have to remember to set the G/A limit yourself?

Cheers
Chris
« Last edit by Christo van der Loo on Sun, 14 Jun 2015 05:01:04 +0000. »
Member
Registered: Oct 2014
Posts: 341
What about pushing TOGA? Does that only command LIMIT THRUST if not TO or GA thrust limit?

On older 737 you had to manually select the N1 limit if not set via FMC mode switching, so my guess would be manual selection is required in the 747-400 as well.

Not really sure how looking at the 777 helps as it is much newer.
Member
Registered: May 2010
Posts: 843
Quote
What about pushing TOGA? Does that only command LIMIT THRUST if not TO or GA thrust limit?


The 744 behaves much the same as the 777 in regards to initial climb rates, airspeed/vertical speed blends, max thrust limit on second push of TOGA switch, etc.

Quote
Not really sure how looking at the 777 helps as it is much newer.


True. The general "philosphy" is much the same, but the 777 may refine or extend the logic or compute it in a different way.

As with everything in PSX, observing the behaviour is only one step towards knowing what causes the behaviour. e.g. if landing flaps are the trigger, "flap position" has to be defined. Sometimes system behaviour is based on first flap group to reach x position, sometimes it's based on inboard TE flap position. Sometimes it's based on flap handle position. Sometimes there are time delays built in (which may make the behaviour seem it is linked to one thing, but it's not). Is the logic present in all three Flap Control Units? etc.

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