RTO for PWS caution

[RMC]

Just relating to the low speed abort regime the Boeing 787 manuals are clear that below 80 knots a stop is executed for a PWS warning ie “windshear ahead”. Therefore (in the absence of any other cues) a take off should be continued for a PWS caution “Monitor radar display”.

Anyone got a different take on this. Have seen some confusion about this on the threads about F/O calling stop.

[ScepticalOptomist]

Above 80kts and prior to V1 we would stop for a PWS WARNING - as per the manuals.

PWS Caution is inhibited from 80kts.

PWS Warning inhibited from 100kts until 50’.

For a PWS Warning WINDSHEAR AHEAD I would expect to stop.

You wouldn’t get your caution (Monitor Radar Display) after 80kts. Continue the takeoff and if after 50’ you have a wind shear WARNING do the wind shear escape procedure.

[RMC]

Thanks for the reply SO. In the high speed regime we are agreed. My question is really about what happens below 80 knots as some manuals suggest aborting only for a PWS WARNING (whereas the generic take off brief we use talks about any MCS WARNING or CAUTION).

[ScepticalOptomist]

Ahh yes, I see what you’re saying.

I’ll have to check, but I thought the PWS CAUTION will only light up the Master Caution light with no aural.

Does this satisfy the requirement to abort the takeoff? I’d be conservative and nominate to stop in the low speed regime.

I will pose this question to our training department and see what their interpretation is.

[wiggy]

Ex T7 and can’t speak for the 78 or any current teaching so just a IMHO but as commander my thinking on this was that I wouldn’t want to be continuing towards potential WS if I still had the option of a low speed abort.

I’m sure other equally or much more valid/current options are available.

[ScepticalOptomist]

Our training department has basically expressed the view that you would most likely stop in the low speed regime, however you would be still in accordance with the SOPs should you choose to continue.

I would rather stop and know that I won’t encounter WS, rather than continue and possibly encounter WS.

[safetypee]

Sceptic… , :ok: an enlightened operator.

No so long ago, pre technology, windshear training was based on observation, ‘know before you go’, if you don't know, don't go.
Nowadays, with technology designed to assist, but is used as the arbiter, decision maker, we devolve our responsibility to a machine - we are cognitive lazy - we dislike thinking.

The original training - required assessment of the environmental conditions with bias towards not departing in marginal conditions; this can be related to a low speed abort - don't go. The training also considered higher speed regions, windshear - a surprise (how do you detect it), and on the balance of risk - a judgement of outcome, thinking (an informed guess), was to continue the take off.
The balance of risk involved that of stopping from high high energy, and continuing through the conditions where increasing aircraft energy was beneficial. This also considered head / tail wind shear and crosswind, some situations which helped, others not so; the pilots didn't know.

Technology may not know any better; possibly in some situations, but neither may we know what the machine knows unless we think about this before we decide to takeoff.
Understand the limits of both human and the machine, and the benefits of them together

The OP (as with most SOPs) ‘cans’ the situation; assumed conditions, awareness, precautions.

A thought - if you get a PWS caution, what was the takeoff power setting ….

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC00-54.pdf

[RMC]

Thanks for the replies. Irrespective of the literal interpretation it looks like we all agree the sensible option is a stop below 80 knots. If you elected to continue and ultimately had a lawyer asking “on what basis did you decide continuing was the safest option” you would not be well placed.

[Fursty Ferret]

There's a bit of discussion about this in the FCTM. From what I understand, the slow acceleration caused by windshear may leave you much further down the runway than your performance figures allowed for at speeds close to V1.

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[vilas]

PWS is a warning for what is a few miles ahead. It won't affect your acceleration.

[FlightDetent]

Add to that what vilas points out, and we seem to have reached the same conclusion as these guys:

For the WARNING

https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/440x80/wshr_bb51691c06767f3ead66627c329877c0fe54b719.png

The ADVISORY (monitor WXR display) is not apart of the abnormal procedures at all, just crew's responsibility.

Sounds legit to me.