Autopilot Issue during Landing

Whenever I try to land using autopilot (both negative vertical speed and APPR) at any airport, the autopilot apparently tells the plane to go in extreme directions as whenever i generally am about midway through the landing, my plane (737-700) pitches UP and sometimes DOWN, resulting in my direction misaligning with a stall and/or crash. i also tried this on a 747 and it did the same thing. So this is for multiple planes (maybe every). This is an issue as this makes landing a nightmare. Is there a solution? (it might just be me idk)

It would be helpful to see a video of this happening.

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Like @BennyBoy_Alpha has said, it would be helpful to see a video of this. I use the APPR system fairly regularly, and never have issues. Is it possible you are going too slowly? In my experience, optimum airspeed for the APPR system to have a successful landing is between 150-135kts.

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I can’t post videos so I need to explain by screenshots but yes here’s an example. In the first screenshot I turn to final approach at KSNA at 3500 ft with flaps about halfway down.

Right afterwards slow down to landing speed (140kts). The second screenshot is when I activate APPR.

I turn but then immediately the nose drops down (3rd)

thus resulting in a crash (4th).

This is what I mean by the autopilot issue.

You seem to have spoilers deployed and flaps at a low degree… Maybe that contributed. I will say the autoland system is wonky tho

@slider42 Can you share the replay instead - you can do so via this link:

@slider42 Welcome to the community!

I agree with the others that a video would help to understand. Or perhaps even some more instrument information.

But for now, your speed does look low for the flap setting and as @Thomas_Cunningham said it looks like your spoilers are deployed.

(edit: After writing this I notice your speed is indeed displayed and remains above 140kts, but it would be useful to see your pitch angle changes through the manoeuvre and appr status lights)

I tried your approach, the 20R ILS starting from 3500 feet AGL, and at 140kts with flaps 15(?), trying to follow your same situation as best as I could understand. Though the angle of attack was a bit high because of the low speed for the flap setting, it still captured the LOC and GS and made it to the runway. (I also had tried adding spoilers, but the auto throttle kept the airspeed up ok)

I’m wondering what happened when the nose dropped? Were you showing green for the LOC and GS capture?; was your airspeed holding at least at 140kts?

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Based off of just your sceenshots, I agree with what @adit and @Thomas_Cunningham have said in that your flaps are very much not at the right settings for what your speed is. If you want help in that area, here is a website I find useful which provides the flap speeds on each aircraft in IF.

I’m also not sure why you have your spoilers deployed either. Would you be willing to provide an explanation on the reasoning behind that?

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I tried again under your flight conditions, and I was able to make it pitch up, stall, and lose control, if I had even -2000 set on my VS when I engaged APPR.

So when I engaged APPR, if:
1)VS was first set to 0, I had a smooth transition to APPR mode
2)VS was set at, say, -2000 (maybe less would cause it too), I had abrupt pitch up causing a stall when engaging APPR.

With flaps 15 and IAS 140kts, it was close enough to stall speed that the aircraft couldn’t stay flying with an abrupt pitch up.

Note that I was below and waiting for the GS to come down to my altitude.

To keep it simple: Intercept the localizer at an angle of no more than 30° as any more will cause oscillations that the autopilot will be slow to control. I usually intercept around 180-170 knots aircraft depending. Have the aircraft ready for the approach way ahead of time, any last minute missteps will lead to unstable approaches. In the 737 I usually intercept with 170kts Flap 15, always trim the aircraft until the magenta line is gone. This insures that you are trimmed properly for the airspeed being flown. With this typically, APPR will transition to the localizer smoothly more or less. When I take over manually, always insure you calibrate prior, if not you could see a dramatic nose drop or incline leading to a stall scenario seen here. When slowing the aircraft down on APPR, do it slowly. Too fast with dramatic flap changes will lead to vertical oscillations on the approach which could also lead to a stall. Practice on solo in different aircraft and see the various aircraft performance guides on the forum to get ballpark data for approaches and landings. Hope this helps!

You’ve just described what you did wrong.
Flaps 15 and 140 kts IAS does not match resulting in a AoA which is too high (over 15 degrees) and therefore resulting in a stall.
You should set your flaps to 30 at that speed to avoid this.
Hint: there’s a little chart with airspeeds and flap settings under the landing gear lever telling you what flap setting to use at which speed. Try to use it and adjust your speed accordingly to the chart.
Also I do not reccomend to use the spoilers when on final. Slow down to 190 kts before entering the red cone and intercept at around 180 kts then slow down to approach speed which is around 140/145 kts with a slight headwind. Trim the aircraft if needed and you should be good.

140kts is way to slow for Flap 15. 140kts is final approach speed so should be set when at Flaps 30.

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I’m not completely certain, but it appeared the OP was identifying mostly a pitch problem while using APPR, both in the opening text and screenshots. So I was trying to focus on that.

This misses the point. The issue is whether sudden pitch changes when engaging APPR are the key problem, and if so, whether one can predict what causes it, and avoid that cause. While I agree 15 flaps and 140kts pushes the AoA, resulting in a narrowed safety margin, I was still able to fly and land with these conditions at MLW. I was flying with these conditions to reproduce the described scenario as this was an important clue from the OP. Flaps at 30 will not avoid the pitch up issue.

I know. Please read the reasoning already described. Again, the focus is sudden pitch change when engaging APPR. Flying at a higher speed or with more flaps will not eliminate the sudden pitch change, if the other conditions causing the sudden pitch change are met.

edit: I’ll just clarify, the condition for sudden pitch up when engaging APPR appears to be, you are below the glide slope and you’ve entered a significant negative value into VS just before engaging.

thanks guys for the tips. ill try these now.

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@adit I know what the OP was referring too, I explained what I do to set up for a stable approach from all angles to cover everything that could lead to loss of control. Whether that be lateral or vertical instability.

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APPR tends to do a thing when if you activate it while below the glideslope and then fly into the glideslope it will send your nose skyward which will 100% cause a stall and crash if you airspeed is too low. If you’re doing any less than 160 knots activating that APPR button is essentially off limits UNLESS you activate it close enough to the glideslope and on the localizer that it engages immediately and thus does not pitch up as violently (if you are close enough to glideslope but off on localizer it will swing towards the localizer violently causing the same issue). My strategy is to intercept 8-10 miles out at 170-180 knots and as soon as I activate it to drop the speed AP to 160 knots. This helps guide the APPR down towards the glideslope (make sure you have enough flaps when you do this, though).

After repetitive tests on 737-7 since yesterday, I found it only does this if you have a good amount of negative VS set when approaching below the glideslope. If zero VS or somewhat positive, it was very smooth.

It was completely ok even with the 140kts and flaps 15 (being as close to stall as it was), as long as VS was not less than zero going in to hitting APPR.

I couldn’t get being off the localizer to make any difference (maybe I need to go back and test if I was far enough from the localizer, but I definitely wasn’t aligned with it)

edit: I was just trying the same kind of tests on the A321 and getting somewhat similar results (though maybe a bit less smooth, not too much though). I had flaps at setting 2 with a low speed of 160kts flying level to intercept. Only thing that guaranteed nose would jerk up suddenly when below the glide path, was again a negative VS setting (a got a stall warning).

As far as localizer intercept from “illegal angles” the thing it didn’t seem to like was hitting the APPR when tuned to GPS rather than NAV (it turned in the wrong direction).

I swear I have had close calls in all these instances but maybe I’m just bad

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Part of the reason I was experimenting, is because I haven’t been clear in the past on exactly what to avoid doing, because I’ve also had the abrupt transitions frequently in the past. What I don’t know though is when, if any, updates were made since my original impressions of APPR performance were formed.

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Sorry, I guess I was just trying to focus on what I didn’t understand myself. I think your recommendations were great.

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