Boeing AP/ATHR

[vilas]

Helo everyone
A question for B737 800 guys​​​​​​.During ILS approach what's the minimum altitude that AP can be kept on? Does the auto throttle need to be switched off along with AP? Is the procedure same in B777?

[FlyingStone]

Minimum use height (in any autopilot mode) is 140ft for 737-700/900/900ER, 138ft for 737-600 and 158ft for 737-800. That's for aircraft with EASA AFM, it might vary for other authorities.

Boeing recommends manual flight = manual thrust for all of their non-FBW models, except in climb (where thrust is fixed). I believe for FBW this isn't the case and A/T can be kept on.

Autothrottle use is recommended during takeoff and climb in either automatic or manual flight. During all other phases of flight, autothrottle use is recommended only when the autopilot is engaged in CMD.

[vilas]

Thanks a lot. Boeing FBW is also speed stable unlike Airbus. Is there an autotrim in Boeing FBW like Airbus?

[rudestuff]

Some surprising answers there! In the 757 and 767 if you've got 3 APs engaged you can leave them on all the way to the runway... With 1 AP you can leave it engaged until 180ft. Likewise, Boeing philosophy is that if the AP goes off the AT goes off as well.

[Occy]

At my company for RNP and ILS/GLS approaches it’s 50ft RA.

[vilas]

How serious is this FCOM and FCTM information?and is it followed at all?

[Banana Joe]

The stabilizer is automatically trimmed only when doing an autoland for which you need to have both AP engaged by 800 ft RA.
​​​​​​For manual landing you only keep one AP engage and flare won't arm and there won't be any automatic nose up trim.

As for disconnecting the AP 1 or 2 miles in, that is only a recommendation.

[Vessbot]

Originally Posted by Banana Joe

The stabilizer is automatically trimmed only when doing an autoland for which you need to have both AP engaged by 800 ft RA.

So long as we're talking flight with AP on, isn't the stabilizer automatically trimmed the entire time?

[Banana Joe]

For autoland, the AP trims automatically a nose up trim bias for the flare. If you disconnected the AP and continue to land, you would need to apply a positive nose down pressure on the yoke to remain on the glide.
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[ImbracableCrunk]

50' for manual landing, after touchdown for autoland. (FAA)

[vilas]

Originally Posted by Banana Joe

For autoland, the AP trims automatically a nose up trim bias for the flare. If you disconnected the AP and continue to land, you would need to apply a positive nose down pressure on the yoke to remain on the glide.
​​​​​​

In B737 are both APs not engaged for every ILS approach? In Airbus irrespective whether the landing is manual or automatic both APs are engaged as SOP.

[FlyingStone]

No, as engaging second autopilot will cause the autopilot to trim slightly nose up at 400ft RA to provide "flare spring bias". This enables the aircraft to slighly pitch up in the event of autopilot disconnect/failure. This obviously wouldn't be desired when manual landing is planned, so single autopilot is used for approach in that case.

[ImbracableCrunk]

Originally Posted by vilas

In B737 are both APs not engaged for every ILS approach? In Airbus irrespective whether the landing is manual or automatic both APs are engaged as SOP.

I've flown for two 737 operators, one in Asia, one in the US. Asian carrier assumed autoland with a manual landing as a possibility. SOP was to always arm both autopilots.

The US carrier assumes manual landing and the SOP arms one autopilot.

[Matey]

The engagement of both autopilots for a planned manual landing was a no no at my employer pre retirement. This is due to the aforementioned up trim applied at 400ft. Not so much of an issue on the -800 with a longer moment arm and more elevator authority, but it is definitely more of a handful on the shorter -500 upon autopilot disegagement below 400ft potentially destabilising the approach at a late stage. The only reason to do it was to keep an autopilot engaged for a go around., whereas on a single autopilot approach, pressing TOGA causes autopilot disengagement.

[oceancrosser]

Why would anyone want to fly, single A/P below 200’?

[Mansfield]

Back in 2006, there was a short presentation given by Captain Mike Bryan of Boeing at their operators' symposium, in which he argued for removing the recommendation against manual flight with the autothrottle engaged from the FCTM. He cited a number of advantages, including no longer needing a speed additive due to winds, fuel savings, reduced workload, etc. As I don't have a current FCTM, I don't know what Boeing has actually done. However, my operator's standard policy is to keep the autothrottles engaged throughout the entire flight. Obviously, this is a requirement for a HUD CAT III landing, as we fly those approaches with the autopilot disengaged from the final approach fix to landing, while keeping the autothrottles engaged. In any other case, our US Op Specs require the autopilot to be disengaged by 50 feet below the MDA, and never lower than 50 feet RA. We are not trained nor authorized to use the autoland feature, thus we never engage two autopilots at the same time.

That said, one of Captain Bryan's cited disadvantages to a new policy was pilot proficiency with manual throttles. Sure enough, about 3 months after I got on the airplane, I picked up a ship with the autothrottles deferred. Having crossed the ocean dozens of times before autothrottles, and several times since with them deferred, I thought no big deal. Next thing I knew we had levelled off at 7000 feet and the wind noise was winding up as we passed 290 knots. Now I generally disconnect the autothrottles for all visual approaches, as we always did in previous airplanes, just to keep engaged with throttle use. Funny how quickly we become complacent, as smart as we think we are.

[FlyingStone]

Originally Posted by Mansfield

Obviously, this is a requirement for a HUD CAT III landing, as we fly those approaches with the autopilot disengaged from the final approach fix to landing, while keeping the autothrottles engaged. In any other case, our US Op Specs require the autopilot to be disengaged by 50 feet below the MDA, and never lower than 50 feet RA. We are not trained nor authorized to use the autoland feature, thus we never engage two autopilots at the same time.

I'm always fascinated why some airlines prefer to have its pilots fly manually in extremely marginal weather conditions with HUD rather than use autoland, where available.

Do you perhaps know why your employer chose to do so?

[Mansfield]

I don't really know the entire genesis of it. I know that the program manager around the time of the 737NG acquisition was (is) a former F-15 pilot who was very enamored with the HUD capability offered on the NG. And, to be fair, rightly so...it is a very powerful tool. I suspect that a certain amount of the impetus toward HUD was that a) it was capable of CAT III fail-passive, and b) in order to use the HUD for CAT I or CAT II, and then autoland for CAT III, you'd have to add a bunch of training time to all blocks of training. I believe there was an effort to add autolanding to the program several years ago, but that was rejected for the above reasons.

That said, I have done both...fail-operational autoland in the 767/757, fail-passive autoland in the MD80 (), and fail-passive HUD landings in the 737NG. They are all good options, INMHO. And we can fly single engine CAT III approaches on the HUD, which I don't think would be an option with autoland on the NG. Of course, feel free to calculate the statistical probability of needing that capability here in the States. We get some relief on alternates from it, but big deal.

[Matey]

The 737NG with fail operational autopilot is certified for planned single engine auto land to CAT3A minima

[Check Airman]

Originally Posted by FlyingStone

I'm always fascinated why some airlines prefer to have its pilots fly manually in extremely marginal weather conditions with HUD rather than use autoland, where available.

A very large airline in the US does it because it’s cheaper.