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Author Topic: TO/GA Go-Around Airspeed  (Read 910 times)

FreightDog767

  • Join date: Aug 2020
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TO/GA Go-Around Airspeed
« on: Sun, 23 Aug 2020 02:32 »
Hi Hardy,

Apologies if this has already been brought up on this forum, but as I was studying for 747-400 transition, I came across this blurb in FCOM Vol. 2 about the TO/GA sub-mode during a go-around (which is also echoed in the CBT module):

"With the first push of either TO/GA switch:

...if current airspeed remains above the target speed for 5 seconds, the target airspeed is reset to current airspeed, (to a maximum of the IAS/MACH window speed plus 25 knots)"

Having tried this in PSX, I note that the aircraft does not reset the speed; the target remains at whatever is set in the IAS/MACH window, even if the aircraft accelerates beyond it for more than 5 seconds.

Any thoughts on this? Thanks!



Hardy Heinlin

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Re: TO/GA Go-Around Airspeed
« Reply #1 on: Sun, 23 Aug 2020 03:33 »
Hi,

I can't reproduce this problem. My TO/GA pitch mode maintains VREF+25 while the magenta speed bug sits on VREF.

Do you expect the speed bug to move higher? This won't happen. Only VNAV can move the speed bug.


Regards,

|-|ardy

FreightDog767

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Re: TO/GA Go-Around Airspeed
« Reply #2 on: Sun, 23 Aug 2020 04:00 »
Hi,

I can't reproduce this problem. My TO/GA pitch mode maintains VREF+25 while the magenta speed bug sits on VREF.

Do you expect the speed bug to move higher? This won't happen. Only VNAV can move the speed bug.


Regards,

|-|ardy

Apologies, no, I don’t mean the command speed bug. I mean that the way the FCOM is written, the airspeed should maintain current bug speed above the IAS/MACH window, up to a maximum of bug+25. Not sure it’s designed to specifically accelerate the jet to bug+25 without first attempting to maintain the current speed above the bug. 

For example, if I’m bugged to 150 knots and press TO/GA, the jet should first attempt to maintain 150 knots. If I was a bit fast and maintain 160 knots for 5 seconds, the jet should attempt to maintain 160, not 175.

I’ll try again to reproduce it tomorrow. Thanks for the response.  :)


Hardy Heinlin

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Re: TO/GA Go-Around Airspeed
« Reply #3 on: Sun, 23 Aug 2020 11:39 »
Are you using a joystick that is stuck in a nose-down position?

I see no problem here in my PSX software code and in the sim behaviour. It does this:

In the moment of the TO/GA push on approach, the system sets its initial target speed; for this purpose the system copies the current aircraft speed or the MCP speed, whichever is higher.

If, in this moment, the current aircraft speed is already higher than MCP+25, the initial target speed too will be higher accordingly. E.g. when you push TO/GA while the current speed is 200 and the MCP speed is 100, the target speed won't drop to 125; it will remain 200. That's the logic in PSX.

But if you push TO/GA while the current speed is less than MCP+25, the target speed may rise after the next five seconds.



« Last edit: Sun, 23 Aug 2020 12:19 by Hardy Heinlin »

Hardy Heinlin

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Re: TO/GA Go-Around Airspeed
« Reply #4 on: Mon, 24 Aug 2020 00:05 »
Does anybody disagree with my logic quoted below? (I call this logic "version A".)

If, in this moment [TO/GA push], the current aircraft speed is already higher than MCP+25, the initial target speed too will be higher accordingly. E.g. when you push TO/GA while the current speed is 200 and the MCP speed is 100, the target speed won't drop to 125; it will remain 200. That's the logic in PSX.

I can't remember if the above logic (version A) has been confirmed on the real deck or big sim. But I can recall that I also tested the following logic (I call that one version B):

Version B: "If, in the moment of the TO/GA push, the current aircraft speed is already higher than MCP+25, the initial target speed will be MCP+25."

The (cumbersome) consequence of version B is this:

Assume the aircraft speed is 190, MCP is 130, on final approach with flaps 10, at -700 fpm. Now push TO/GA.

The elevator commands 155 KIAS, and the A/T aims at 2000 fpm. But these 2000 fpm are quickly reached without any thrust increase because the aircraft pitches up to reduce the speed from 190 to 155. This pitch-up alone increases the climb rate already. Only when 155 is reached, the A/T would increase the thrust to maintain 2000 fpm. And if the respective flaps for 155 (e.g. flap 20) are not extended yet, you even need to extend the flaps during the TO/GA pitch-up. This process looked cumbersome to me. So I implemented version A.

In PSX, the 5-second timing logic only operates when the initial target speed starts below MCP+25.


|-|ardy

FreightDog767

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Re: TO/GA Go-Around Airspeed
« Reply #5 on: Mon, 24 Aug 2020 02:55 »
Does anybody disagree with my logic quoted below? (I call this logic "version A".)

If, in this moment [TO/GA push], the current aircraft speed is already higher than MCP+25, the initial target speed too will be higher accordingly. E.g. when you push TO/GA while the current speed is 200 and the MCP speed is 100, the target speed won't drop to 125; it will remain 200. That's the logic in PSX.

I can't remember if the above logic (version A) has been confirmed on the real deck or big sim. But I can recall that I also tested the following logic (I call that one version B):

Version B: "If, in the moment of the TO/GA push, the current aircraft speed is already higher than MCP+25, the initial target speed will be MCP+25."

The (cumbersome) consequence of version B is this:

Assume the aircraft speed is 190, MCP is 130, on final approach with flaps 10, at -700 fpm. Now push TO/GA.

The elevator commands 155 KIAS, and the A/T aims at 2000 fpm. But these 2000 fpm are quickly reached without any thrust increase because the aircraft pitches up to reduce the speed from 190 to 155. This pitch-up alone increases the climb rate already. Only when 155 is reached, the A/T would increase the thrust to maintain 2000 fpm. And if the respective flaps for 155 (e.g. flap 20) are not extended yet, you even need to extend the flaps during the TO/GA pitch-up. This process looked cumbersome to me. So I implemented version A.

In PSX, the 5-second timing logic only operates when the initial target speed starts below MCP+25.


|-|ardy

I'm actually really interested in trying this in the sim; I'll get plenty of opportunity, I'm sure.

Now, speaking as a 757/767 driver (admittedly with zero experience on the 747 at this point, but a Boeing is typically a Boeing), the FD *will* command a very significant pitch-up in the "TO" submode if you're faster than the V2+25 limit before VNAV is engaged. Occasionally, you'll see the FD command a pitch-up of 25+ degrees in order to slow you back to V2+25, though we as pilots would limit the pitch attitude to 20 degrees and fly "through" the FD and let the jet accelerate until VNAV is engaged at 1000'. The 757 was especially prone to this, but a light 767 will do the same. Slower than normal rotation and a light jet is a recipe for it.

I can count on one hand the number of times I actually did a go-around in the 757/767 in real-world conditions in my 10 years on it, so I can't speak much to that.

To answer your previous question, I'm not actually using a joystick at all. For my purposes, a keyboard and mouse does just fine. I was seeing this occur with the autopilot engaged.

Thanks for looking into it! Oh, and something to keep in mind, we wouldn't actually set the airspeed bug down to Vref+5 in one fell swoop; to protect against getting distracted and getting slow, we bug down to the next flap maneuvering speed once the next flap setting is called (or, I often wait until they're set, but just my practice). So, having to extend flaps during a go-around at a higher than normal speed is very unlikely.


Hardy Heinlin

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Re: TO/GA Go-Around Airspeed
« Reply #6 on: Mon, 24 Aug 2020 04:52 »
I understand that in normal ops you won't have large differences between current speed and bug speed. My examples are exaggerated just to show how idiot-proof the logic can be in non-normal situations. Such non-normal situations sometimes show effects that make no sense although the sim programming agrees with the (sometimes vague) FCOM text and other documents.


|-|ardy

dhob

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Re: TO/GA Go-Around Airspeed
« Reply #7 on: Mon, 24 Aug 2020 08:00 »
From the FCOM:

When a go-around is initiated,  the  command speed is the MCP IAS/Mach  window  or current  airspeed,  whichever is  higher.  If the airspeed  increases and remains above  the initial target airspeed for five seconds,  target  airspeed resets  to  current airspeed  to  a  maximum  of the IAS/MACH window  speed  plus 25 knots. If airspeed  at  initiation of go-around  is greater than  IAS/Mach  window  plus 25  knots, that  speed is maintained.

Jeroen Hoppenbrouwers

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Re: TO/GA Go-Around Airspeed
« Reply #8 on: Mon, 24 Aug 2020 10:04 »
If airspeed  at  initiation of go-around  is greater than  IAS/Mach  window  plus 25  knots, that speed is maintained.

What speed is maintained?

Given that it should be something not stated by the previous statement, which is window+25, it must be the airspeed at initiation of go-around, I think.

So if you are really fast, you remain really fast.


Hoppie

Hardy Heinlin

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Re: TO/GA Go-Around Airspeed
« Reply #9 on: Mon, 24 Aug 2020 12:42 »
Ah, thanks for the refresher, Dhob. So PSX is correct. It wasn't just my theory; it's indeed in the FCOM.

Hoppie, I read the word "that" as a reference to the part directly before the word "that":

If airspeed at initiation of go-around is greater than IAS/Mach window plus 25 knots, that speed is maintained.


|-|ardy

FreightDog767

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Re: TO/GA Go-Around Airspeed
« Reply #10 on: Mon, 24 Aug 2020 18:12 »
Oddly enough, that section dhob posted isn't in our Company's FCOM. Good to know!

But, that brings me back to my original question: If the bug is set to 150, and I engage TOGA at 160 (and it remains at 160 for more than 5 seconds), the TOGA pitch sub-mode should pitch for 160, not 175, yes? I notice that on PSX, the aircraft accelerates to bug+25.

Hardy Heinlin

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Re: TO/GA Go-Around Airspeed
« Reply #11 on: Mon, 24 Aug 2020 18:53 »
I notice that on PSX, the aircraft accelerates to bug+25.

This is because it pitches up smoothly, and the thrust increase might be slightly faster, so there may be a slight speed increase during the pitch-up rotation. The rotation takes at least 5 seconds. So there you'll have the first 5-second reset.

Try a manual, faster TO/GA rotation, so that your speed remains on the bug during the thrust increase. Then engage the A/P and watch: You'll see the speed will remain on the bug.

FreightDog767

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Re: TO/GA Go-Around Airspeed
« Reply #12 on: Mon, 24 Aug 2020 21:15 »
I notice that on PSX, the aircraft accelerates to bug+25.

This is because it pitches up smoothly, and the thrust increase might be slightly faster, so there may be a slight speed increase during the pitch-up rotation. The rotation takes at least 5 seconds. So there you'll have the first 5-second reset.

Try a manual, faster TO/GA rotation, so that your speed remains on the bug during the thrust increase. Then engage the A/P and watch: You'll see the speed will remain on the bug.

Thanks Hardy, I’ll try it.  :)

Toga

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Re: TO/GA Go-Around Airspeed
« Reply #13 on: Sun, 25 Oct 2020 11:14 »
I have been practising the RNAV into Kathmandu - the saved scenario. During the Go around VNAV will command 180 knots once activated as there's a SLP to get around the corner and beyond KT604 or something like that but I'm finding if the aircraft levels at 11,000 before that point it will increase its speed beyond 180 to CRZ speed and then once passing over the waypoint the speed will then be reselected to 180. Is this correct? Seems a bit of a gotcha.

---------------------------^^^^I think the above was my fault. Didn't happen second time around.

I'm not that familiar with the Boeing side of things but I haven't found a VNAV page for the go around which opens up confusion at times.
« Last edit: Sun, 25 Oct 2020 11:38 by Toga »

Hardy Heinlin

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Re: TO/GA Go-Around Airspeed
« Reply #14 on: Sun, 25 Oct 2020 13:03 »
Didn't happen second time around.

It doesn't happen when you use the legacy FMC. My NG model should behave the same, but I forgot to re-arrange the priorities of the speed limit selection at the T/C. Unlike the legacy FMC, the NG FMC sets the best hold speed at T/C if the missed approach includes a hold -- this happens in the real NG and in PSX. But PSX shouldn't do this as long as a lower waypoint speed constraint is still active.


|-|

Toga

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Re: TO/GA Go-Around Airspeed
« Reply #15 on: Sun, 25 Oct 2020 16:58 »
Try loading the Kathmandu RNAV situ you provided and fly the approach with the NG FMC. I keep having this problem:



It happens even if i level at some point during the turn before the KT604 WPT

Hardy Heinlin

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Re: TO/GA Go-Around Airspeed
« Reply #16 on: Sun, 25 Oct 2020 17:22 »
Did you read my last comment :-)

Toga

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Re: TO/GA Go-Around Airspeed
« Reply #17 on: Mon, 26 Oct 2020 14:14 »
Yes i did but was confused as to what you meant by this: 

Quote
this happens in the real NG and in PSX. But PSX shouldn't do this as long as a lower waypoint speed constraint is still active.

I read it as the lower waypoint speed - 180 kts @ KT604 - should be active until it's passed - like i see with the legacy FMC.

Are you saying the NG FMC will overwrite the speed restriction at ToC?

Also why does changing the Vref between flaps 25 or 30 change the flaps up / 1 / 5 speeds on the speed tape?


T
« Last edit: Mon, 26 Oct 2020 14:38 by Toga »

Hardy Heinlin

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Re: TO/GA Go-Around Airspeed
« Reply #18 on: Mon, 26 Oct 2020 14:50 »
Are you saying the NG FMC will overwrite the speed restriction at ToC?

The real NG FMC won't do this. The PSX NG FMC does it (my fault) because I forgot to modify the speed limit priority logic at T/C.

The real NG FMC only activates the CRZ phase when all missed approach constraints are passed. Before this happens, the CLB phase remains active; the ACT CLB allows the FMC to set the lowest of all speed limits. When the CRZ phase is active, the FMC ignores waypoint speed limits as speed limits are invalid when in CRZ.


Quote
Also why does changing the Vref between flaps 25 or 30 change the flaps up / 1 / 5 speeds on the speed tape?

This is an option (British Airways) which can be installed on Instructor > Model > FMS
by deselecting "Flap speeds refer to VREF30". See checkbox tooltip text for more info.


|-|

Toga

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Re: TO/GA Go-Around Airspeed
« Reply #19 on: Mon, 26 Oct 2020 15:07 »
Ok understood. Thanks Hardy!