Takeoff: At 65 KIAS, THR REF > HOLD

[Hardy Heinlin]

I've read reports of aircraft departing in extreme wind conditions. This may have a nice side effect:

When you're in the takeoff position and you push TO/GA, the A/T engages in THR REF and will try to set the throttles exactly on the selected thrust limit.

If your IAS tape fluctuates near 65 KIAS in this moment, it may peak above 65 before the thrust limit is perfectly set because at 65 the mode changes to HOLD.

I just rechecked if the 65 is really airspeed and not groundspeed. The AOMs say KIAS.

If your headwind is 66 KIAS at the takeoff position, TO/GA will not work a all :-) The throttles will immediately stop moving.


Cheers,

|-|ardy

[Jeroen D]

If your headwind is 66 KIAS at the takeoff position, TO/GA will not work a all :-) The throttles will immediately stop moving.

Looks like the throttles have more common sense than the pilot pushing a TOGA button in a 65 KIAS headwind.

Jeroen

[Phil Bunch]

A slightly related comment - I've often wondered what it would be like or if it is technically possible to land in very high headwinds.  A Cessna might be able to land at say 5 mph ground speed if the head wind is (very roughly) 50 or so MPH.  Would a 747 require a 130 mph headwind to be able to land at 0 mph groundspeed?  

Crosswind upper limits are established.  Are there any legal or other limits on landing in headwinds?  I would think that the *variability* of winds would be the most difficult aspect of windy landings rather than the absolute level of the wind.

I recall one 737 landing where the pilot abandoned our high and variable wind-plagued landing roughly 100 feet from the ground, choosing to go around and try it again.  It wasn't a pleasant situation as a passenger - the aircraft was flopping around quite a lot in various axes as we tried to land.  It seemed like the crosswind was pushing the wing up in an erratic fashion as it varied.  At one instant we were doing well and flying in a stable manner, but the next second the wind seemed to lift a wing and we were quite noticeably "tilted".  Probably the actual angles were not very great and not a real threat but they seemed like it to us passengers.

[Jeroen D]

A slightly related comment - I've often wondered what it would be like or if it is technically possible to land in very high headwinds.  A Cessna might be able to land at say 5 mph ground speed if the head wind is (very roughly) 50 or so MPH.  Would a 747 require a 130 mph headwind to be able to land at 0 mph groundspeed?  

slightly different approach but the effect is the same; small plane landing on a cargo plane with a headwind. Very little relative forward speed between the plane and the ship:

http://www.youtube.com/watch?v=pUdzVnZBaoY

Jeroen

[IefCooreman]

If your IAS tape fluctuates near 65 KIAS in this moment, it may peak above 65 before the thrust limit is perfectly set because at 65 the mode changes to HOLD.

I just rechecked if the 65 is really airspeed and not groundspeed.

Even worse (777 but I presume 747 is the same)

FCTM Note: Takeoff into headwind of 20 knots or greater may result in HOLD before the autothrottle can make final thrust adjustments.

Pretty much the reason why FCTMs call for the monitoring pilot to verify and set the thrust if not set by the autothrottles in windy conditions.

[John Golin]

If your headwind is 66 KIAS at the takeoff position, TO/GA will not work a all :-) The throttles will immediately stop moving.

That explains a lot - I'm glad I saw this post as  I was wondering what was going on

[MEade]

If your headwind is 66 KIAS at the takeoff position, TO/GA will not work a all :-) The throttles will immediately stop moving.

This is also the case in the aircraft I fly; when the INDICATED airspeed reaches 60kts, the autothrottle logic transitions the active mode to HOLD, if the thrust levers have not reached the TOGA detent we get an EICAS caution ENG TLA NOT TOGA.

[jonb]

I've not experienced this personally but did speak to one of our guys who had it coming out of Manchester a few winters ago, so yes, it can happen.

Jon

[Peter Lang]

Today there could be a good oportunity for testing.  

EHAM 050501Z 0506/0612 24015KT 9999 FEW030 BECMG 0506/0509 24023G33KT BECMG 0509/0512 24033G45KT 7000 -RA OVC020 OVC030 BECMG 0514/0516 32035G50KT TEMPO 0515/0517 32045G62KT 4000 SHRA TSRA SCT020CB OVC030 BECMG 0516/0518 30018G30KT 9999 NSW SCT030 TEMPO 0518/0524 32025G38KT 5000 SHRAGS SCT020CB BECMG 0521/0524 31023G35KT CAVOK PROB30 TEMPO 0607/0612 7000 RA -SHRA SCT025 BKN040

Peter

[Jeroen D]

Today there could be a good oportunity for testing.  

EHAM 050501Z 0506/0612 24015KT 9999 FEW030 BECMG 0506/0509 24023G33KT BECMG 0509/0512 24033G45KT 7000 -RA OVC020 OVC030 BECMG 0514/0516 32035G50KT TEMPO 0515/0517 32045G62KT 4000 SHRA TSRA SCT020CB OVC030 BECMG 0516/0518 30018G30KT 9999 NSW SCT030 TEMPO 0518/0524 32025G38KT 5000 SHRAGS SCT020CB BECMG 0521/0524 31023G35KT CAVOK PROB30 TEMPO 0607/0612 7000 RA -SHRA SCT025 BKN040

Peter

I believe it did get pretty rough in Western Europe. Mind you not everybody was experiencing headwinds only. This kind of weather always gets us a few more interesting cross wind landing video s too:

Enjoy: http://www.bbc.co.uk/news/world-europe-17863501

[Balt]

In response to Phil Bunch's question: There is no legal/structural/engineering limit to how much headwind you can land or take off in, at least not for any aircraft I have ever flown. Wind limitations only apply to crosswinds and tailwind (max tire speed). So you can in theory do your 744 landing at walking pace if you have 130kts or so of headwind... the problem you'll in practise see of course is the variability of that headwind, as this close to the ground, it won't be a nicely laminar and steady flow of air coming your way unless you're on an aircraft carrier... now thats an idea! :-)

[Hardy Heinlin]

If your headwind is 66 KIAS at the takeoff position, TO/GA will not work a all :-) The throttles will immediately stop moving.

To be precise, THR REF engagement with the TO/GA switch is already inhibited at 50 KIAS. Those 65 knots are the point where the mode changes to HOLD. The 15 knots difference is probably supposed to prevent flicker effects.


Cheers,

|-|ardy

[mk777]

B777 enters HOLD mode at 84KIAS (or 80 depending on software), I believe its to do with the RTO braking becoming active at 85KAS. AFAIR all boeings since the beginning of time (I can't talk for Mds) have had the warning about take off thrust needing to be set by 60kts ish to protect the TO performance in strong headwinds, and not only to prevent the thrust stopping when HOLD activates.
I wonder if any engineer can comment on any airflow anomalies at high speed if thrust isn't set, perhaps there is an increased chance of a compressor stall in these cases.
All the best.
MK777