Originally Posted by
Uplinker
This seems a weird way of controlling an aircraft with the autopilot flying - a sort of half and half. "I will do this but you have to do that bit".
There was CWS on the 737 that had you hand flying with the autopilot engaged, why not the HOLD mode with autothrottle engaged?
Originally Posted by
Uplinker
I think that Boeing use very confusing terminology: "HOLD" suggests to me that some parameter is being held by the automatics, e.g. altitude, speed; not that, in fact, the auto-thrust has gone to sleep.
What Boeing actually meant was that the pilot must hold the thrust levers with their hand !!
Pilots see HOLD on every takeoff from 65 knots to 400 feet above the airport. So, they should easily know what the term means as it is critical knowledge about what it means for the takeoff both before and after 80 knots.
Originally Posted by
Uplinker
And if, as I understand it, this "HOLD" mode can cause a major elephant trap such as that experienced by the B777 into KSFO where the auto-thrust stayed in HOLD - i.e. asleep - with decaying speed and crashed, then it seems to be very odd 'logic' to me - as a non B777 pilot.
I thought that Boeing updated the logic on this a few years ago to give autothrottle wake-up. Perhaps it is only optional and not all aircraft have the update.
Originally Posted by
maui
1) If FLCH is capable of direct control of V/s in the manner you are proposing, what could an independent VS input contribute that the FLCH cannot?
It would be foolish to try and use FLCH for a desired vertical speed when precise descent rate is required, as it involves too much guesswork when one wants to minimize erroneous descent rates. For example, if you have a glideslope that was fluctuating significantly on approach(as happens sometimes) leading to the aircraft pitching up and down as the automatics chase it, quickly selecting a VS value of 800 fpm(or whatever you deem appropriate) can allow you to continue while returning to a stabilized descent. Then you can make minor adjustments(as low as 100fpm increments), and the aircraft will adjust precisely to the new desired descent rate. Trying this sort of recovery with FLCH would have a high likelihood of chasing a proper descent value as you vary thrust and try to set up the desired, stabilized descent rate. Or maybe VS for a descent to MDA on a non-approach(visual or instrument) where for some reason, vertical nav is unavailable.
VS is precise. FLCH is not very precise (more suitable to a situation such as a descent from ten to five thousand when VNAV is no longer desired and you don't care if the descent is at 2000fpm or 2500 fpm. FLCH is typically used once on vectors for the approach and VNAV is no longer desired. Frequently, you are brought in a bit high by ATC. Therefore, FLCH used on a lot of flights during descent once VNAV is no longer needed as in most cases it quickly brings the thrust to idle. Then you can decide if you need speedbrakes.
VS used much less often in my experience. Maybe if you are low too early, you might use it to reduce the rate of descent. Or one might want to reduce a high descent or climb rate when approaching a level off altitude where there is proximate traffic as this reduces closure rate preventing a TA or perhaps even an RA(select VS at 1000fpm).