THR vs THR REF on a go around (744/8)

[RandomPerson8008]

When would you want to press TO/GA a second time during a go-around in a 747? The FCOM says that the first press gives you THR and commands the TO/GA thrust limit while the second push gives you THR REF.

"Push (on approach with flaps out of up or glideslope captured) –
• activates A/T in THR mode with GA reference thrust limit displayed • selects TO/GA roll and pitch modes
• second push - activates A/T in THR REF mode
Note: With no A/P or F/D active and TO/GA armed for Go Around, pushing a TO/GA switch displays FD, THR, TO/GA, and TO/GA on both PFDs. Thrust adjusts to provide a 2000 feet per minute climb."

Is it accurate to state that the 2000 fpm note applies only when selecting TO/GA withOUT an A/P or F/D engaged? In the past I may have been under the mistaken impression that even with AP/FD on the autothrottle applied thrust for 2000 fpm climb.

[Fursty Ferret]

Caveat - current 787 driver, not 747.

If the FDs or AP are on then the autothrust engages in THR mode for 2000 fpm. Pushing a TOGA switch with FDs off will bring them back temporarily, so yes, you'll get your 2000 fpm thrust target.

[RandomPerson8008]

Ok, so 2000 fpm happens on initial TOGA press regardless of whether or not the FD/AP are on when the TOGA switch is pressed. Thank you.

[Fursty Ferret]

Correct :-)

[RandomPerson8008]

Perfect, the question came up in regard to the windshear recovery maneuver. There has been some debate about whether or not it is prudent to press a TOGA switch a second time if windshear is encountered after already having pressed it once to initiate a go-around. It seems to me that getting THR REF on the second press would be beneficial. On the other hand, in a windshear warning (decreasing performance) event, it is unlikely that 2000 fpm will be attainable so the autothrottle will already be commanding thrust up to the TOGA limit with THR annunciated as the autothrottle FMA. The FCOM procedure does say that "push either TOGA switch" is part of the windshear recovery maneuver, it does not grant leeway to skip that step if TOGA had already been pressed to initiate a go-around.

[Intruder]

YES - push the TOGA button again - it can't hurt! At light weights (below max landing weight), 2000 fpm is easily attained, especially if the landing gear has already been raised and/or if you have the -B5F engines.

[RandomPerson8008]

Thanks Intruder. Unfortunately we don't have the B5F on the -400 at my present employer.

I agree that that the second press of TOGA upon encountering windshear after a go-around has already been initiated seems prudent.

Additionally, perhaps it builds habit patterns to always press TOGA upon activation of a windshear warning. Such habits could be beneficial in the case of a windshear on departure, during which pressing TOGA might not be as instinctive for some pilots as it is during a windshear on approach/landing.

The only negative might be excessive rates of climb after the aircraft is clear of the windshear, but ideally pilots should be able to handle that easily by changing their vertical mode from TOGA to something more appropriate.

[back to Boeing]

It obviously depends if you are flying in manual flight or in automatic flight and whether or not you’ve had the wind shear warning.

In manual flight the wind shear escape manoeuvre clearly state aggressively apply maximum thrust and disconnect the auto throttle. You are merely configuring the flight directors for the escape manoeuvre. a second press of the toga buttons will not achieve anything as you have disconnected the auto throttle.

Also if you identify the wind shear when the aircraft hasn’t you will have to disconnect the system and then achieve the escape manoeuvre. Again the thrust levers will be at maximum. However in this situation the AFDS will not give you escape manoeuvre guidance. That’s up to you.

I don’t think that in autoflight if you have had the wind shear warning that a second press of the TOGA switches will make the aircraft do anything different to what it is already trying to achieve.

The aircraft certainly won’t be attempting to give you 2000fpm if you’ve had the wind shear warning in auto flight.

Having re-read the OM-B Vol 2 it states

“As a rate of climb increases from 600-1200 feet per minute AFDS gradually transitions from pitch to airspeed control etc etc. Which infers that the thrust will be whatever is required to achieve that airspeed control.

just having re-read your post. You absolutely MUST press the toga switches a second time after you have initiated a go around and subsequently encounter wind shear. It is the only way to configure the AFDS system for the escape manoeuvre.

As to your initial question. A single press of the TOGA switches in a normal go around will attempt to achieve 2000fpm. No matter if you are in CMD or FD.

[SaulGoodman]

Also if you identify the wind shear when the aircraft hasn’t you will have to disconnect the system and then achieve the escape manoeuvre. Again the thrust levers will be at maximum. However in this situation the AFDS will not give you escape manoeuvre guidance. That’s up to you.



I agree with you but for clarification: if you press TOGA once before the system has identified windshear you perform a “normal GA” with 2000fpm (unless of course you disconnect the AT and apply full thrust, which you should do in this situation anyway).

If the system then identifies the windshear, all you have to do is press TOGA again to go into the windshear escape maneuver.

[RandomPerson8008]

So, it is not 100% clear to me if the autopilot-flight-director & autothrottle performs differently in windshear escape vs. a normal go-around. I always thought that it did as described by the FCOM regarding 600-1200 fpm pitch to speed transition, etc, but a colleague seemed to challenge my belief. I was under the impression that the AFDS did not behave in the same manner during a normal go-around when no windshear was annunciated.

The follow-up question, then becomes, if the AFDS/authothrottle does in fact behave differently for windshear recovery, what exactly is technically required to for the airplane to shift from normal go around to windshear? Is it the subsequent press of TOGA after windshear is annunciated? If TOGA is already the active pitch and roll mode is the presence of the warning sufficient to activate the windshear flight guidance?

It seems well established that pressing TOGA after windshear is annunciated is required by the FCOM windshear recovery (even during a manual recovery, the FCOM says to press TOGA, then to turn off the autothrottle, as obviously pressing TOGA will automatically re-engage autothtorttle unless someone has turned the arm switch on the MCP off). I'm more interested now in know "why" to help settle the debate.

[safetypee]

Not specifically 747 but similar systems of the era used energy management as a basis for WS GA escape guidance.

Wind-shear detection is generally based on a change in energy, which could be modified by altitude; i.e. earlier trigger at low alt than higher up.

Wind-shear escape guidance (FD or AP), depends on the thrust setting - energy available. Some aircraft systems change from TOGA (reduced 1 or higher 2) to provide absolute Max thrust for the conditions.

The guidance logic balances the need to avoid ground contact by maximising aircraft perf for the thrust available (low alt), and moving away (through) the horizontal effect of the shear; i.e. comparing energy required vs energy available. In downburst conditions the priority is ground avoidance, best climb perf, but with less vertical shear component (more energy available) there is a tradeoff with speed increase to quickly avoid the area, this is also a function of altitude.

These systems differentiate between the normal GA (TOGA 1 or TOGA 2) mode and WS GA; where the latter could increase thrust and use a different guidance algorithm.
System switching would be aircraft dependent - auto thrust and auto WS guidance based on alerting, or manual re-selection of GA / thrust after an alert - which appears to follow Boeings philosophy of manual command, cf #8.

[back to Boeing]

RandomPerson8008

there’s a couple of misconceptions here but I don’t have a huge amount of time to reply at the moment and also the exact logic I am unsure of.

All I have to go on is experience in flying multiple Boeing types and the manuals that are issued to train pilots. But I don’t know much more technically than what is in them.

but firstly what exactly is it that your colleague is saying that you’re wrong on. Just so I can try to give you the correct answer (need to know the exact confusion).

secondly, you disconnect the auto throttle so that the aircraft doesn’t revert to the wrong mode when you go zooming through your missed approach altitude when you’re attempting to recover. Having the thrust levers go back to idle when you’ve just escaped a wind shear will leave you needing new underwear.

But to be crystal clear. Once you have initiated a go around by pressing the toga switches and you subsequently get a wind shear, no matter what mode you are in (manual or auto flight) you must press the TOGA button again to get the aircraft from out of the normal go around mode and in to the maximum performance mode (these are not technical terms). What exactly happens to the flight director logic at this time I’m not 100% sure. But a wind shear warning does not automatically feed anything in to the AFDS. You MUST press toga to initiate the wind shear mode in the AFDS.

[RandomPerson8008]

We were on a approach with full automation on and a go around was exexuted at 800 feet by pressing TOGA due to marginal flight path control (over 5 degree drop in pitch attitude and corresponding reduction in thrust to near idle). Subsequently, at 1000 feet after initiating the go around, an immediate windshear warning occurred. I said "press TOGA". He said, "no, it doesn't do anything to press it again". He didn't press it and the airplane flew through the windshear fine on autopilot with FMA THR|TOGA|TOGA.

All in the sim of course.

[Intruder]

It will "fl[y] through the windshear fine" as long as the downdraft does not exceed the climb rate, and/or you have enough time and altitude. The problem is that you don't KNOW how much time you have, or how intense the downdraft will be. That's why you press TOGA again, to get max performance from the airplane. When through it, press LNAV and VNAV to continue on the Missed Approach. FCOM description of TO/GA switch operation:Push (on approach with flaps out of up or glideslope captured) -
• activates A/T in THR mode with GA reference thrust limit displayed.
Thrust adjusts to provide a minimum 2000 feet per minute climb
• selects TO/GA roll and pitch modes
• activates F/D with no A/P or F/D active
Second push - activates autothrottle in THR REF using GA reference thrust.

[back to Boeing]

We were on a approach with full automation on and a go around was exexuted at 800 feet by pressing TOGA due to marginal flight path control (over 5 degree drop in pitch attitude and corresponding reduction in thrust to near idle). Subsequently, at 1000 feet after initiating the go around, an immediate windshear warning occurred. I said "press TOGA". He said, "no, it doesn't do anything to press it again". He didn't press it and the airplane flew through the windshear fine on autopilot with FMA THR|TOGA|TOGA.

All in the sim of course.

a) if your colleague suggests something which is the safer course (pressing toga again) you just bloody do it.

b) both of you (and I am seriously sorry to say this) have a severe lack of knowledge of the FCOM and the FCTM the questions you are raising are just not acceptable for either of you. Wind shear and the escape manoeuvre is not a question of finesse or technical knowledge. It is a basic manoeuvre that needs to be remembered and understood as well as any other memory item. Probably more so than most.