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A321 - Useless A/THR or v2500 lag

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A321 - Useless A/THR or v2500 lag

Old 5th Feb 2013, 21:27
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A321 - Useless A/THR or v2500 lag

Apart from disconnecting the A/THR or adding a few knots to allow for it, does anyone have any operating suggestions for the A321 in windshear? Anyone have any good techniques to prevent the THR lag?

Flying an approach the other day we had significant undershoot shear. With the loss of airspeed I was surprised by the excessive sink rate which occurred (1600fpm), and the extreme lack of A/THR and/or engine response.

Disconnecting the ATHR we recovered the situation. Truly surprised we did not receive a reactive windshear warning.

It just seemed to lag as the V2500 spooled to its higher N1 to achieve the trust/EPR required.

Last edited by Bula; 5th Feb 2013 at 21:30.
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Old 5th Feb 2013, 21:39
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With the loss of airspeed I was surprised by the excessive sink rate which occurred (1600fpm), and the extreme lack of A/THR and/or engine response.
Excessive sink rate? Pull the stick back. That'll make the (any) ATS respond...I would hope.
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Old 5th Feb 2013, 22:13
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Bula,

Considering you took the trouble to start this thread, you have given us very little information to try and answer your question.

1) What approach angle?
2) Flap setting?
3) What speed, and was Speed selected or managed?
4) AP engaged?
5) What was the wind-component before the speed loss, and roughly how much did it change?
6) EXACTLY what do you mean by "undershoot shear"?
7) Light, medium or heavy weight?

Last edited by Chris Scott; 5th Feb 2013 at 22:15.
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Old 6th Feb 2013, 05:23
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320 series autothrust works really, really well. In older aircraft it was normal practice to turn autothrottles off in anything other than smooth air. Airbus is just the opposite. It works better than most pilots - up to a point. In very gusty conditions, it is still better to turn it off. If you crash, the autothrust doesn't get blamed, the pilots do.

Unfortunately there are a number of airlines out there that require the use of autothrust in their SOP's. If you don't practice and stay proficient, "it won't be there when you need it."

But if your company doesn't allow it, you just have to be ready to take over.
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Old 6th Feb 2013, 11:23
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Bloggs....... A/THR responds very well once in A/Floor


Mr Scott, I don't know how any of that would relate to the thrust responsiveness of the A321, though in good faith, here you go.

1) 2.9 degree
2) Conf Full (27 Degrees)
3) Managed Speed. VAPP = Vls + 7
4) AP off
5) runway 33.
1000' 320/15
700' 280/24
500' 230/28
400' 220/38
200' 300/18
100' 320/7

6) Undershoot Shear, tendency of the aircraft to undershoot the GS due to an initial lack of performance.

7) 75t


USMC seems surprisingly less pronounced in the older fleet MSN 1400 compared to the newer ones MSN 3500.
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Old 6th Feb 2013, 12:10
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My point was that you let the aircraft develop a sink rate of 1600fpm. Unless the speed suddenly dies in the @rse (you "lost" 25 knots over around 45 seconds [600ft at 750fpm, maybe less time because of your increased sink rate]; I wouldn't call that a really bad windshear), the A/THR isn't going to jam on a heap of power in a hurry. Now if you'd pulled back on the stick smartly to stay on the slope/maintain the approach sink rate (dunno what Vls is but say 150=750fpm) the speed would have dropped off at a greater rate and the ATS would have kicked in earlier.

In my machine, a significant undershoot shear corrected by a good pull on the stick would have the ATS (throttles here) jumping out of it's skin to correct the speed reduction.

I'm wondering if your A/THR would have reacted quicker if you'd reacted sooner to counteract the undershoot shear?
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Old 6th Feb 2013, 12:16
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If operating the A321 in wind-shear then use flap 3 and ensure you have 5kts on VLS in the approach page.

Then you should be fine.
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Old 6th Feb 2013, 15:07
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Flying a spankin' new 320 in moderate turbulance (almost every day) I tried a few times to leave everything hooked up. AP and AT. The last time I hit 2000 fpm at 1200 AGL. That was the last time I let the airplane fly in similar conditions. With me flying in the same conditions, I keep it within 200 fpm of target rate of descent, and closer on airspeed.

When conditions are smooth, I can't even come close to the accuracy of the aircrafts systems. But when the going gets rough, the organic stick actuator is better.

I also flew MSN 300-311 3 years ago. But in much more benign conditions. I don['t know how well they would do.

Airbus autothrust is really good. Much better than Boeing, including the 777. But lookup the limitations of the basic aircraft. Then look up the autoland limitations. The aircraft is much more capable with YOU operating the controls than the built in automation.
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Old 6th Feb 2013, 15:47
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According "Uncle Airbus" aka FCOM the autothrust system is an aid you may use to reduce your workload. If you are not satisfied with its performance you have to disengage it. All the "buses" I've flown so far, from MSN 71 to the brand new one last year (5xxx?) fly nicely without the computer messing around with the thrust. So at least for me it works fine.
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Old 6th Feb 2013, 18:04
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Hi Bula,

You seem to have got some real, current pilots on the thread since my request for more data. I retired 11 years ago, and most of my 14 years' A320 experience was on CFM-powered A320s, whose MSNs were in single or double digits. However, after many years we added a bunch of A319s with the IAE engine, while retaining the old A320s.

Flying both types regularly, my impression was that the A/THR-engine combination responded slightly better to windshear on the CFM A320s than the IAE A319s. But the A319 needed less thrust on the approach, which could have made a difference.

What I was trying to establish was if you were likely to have been carrying any extra speed due to GS-Mini prior to your "undershoot shear" (never heard that term before, but think I've got the idea now). If you were, that would make the sudden loss of height unlikely without a substantial downdraft.

You've given me a great clutch of winds, but at what height did you have the shear in question? 600 ft, perhaps? What surface wind had you entered in the PERF page? Finally, roughly what was the minimum speed relative to (a) managed speed and (b) VLS?

BTW, my practice when visual was to disconnect the A/THR if I wasn't using the AP. The throttle levers are a joy to use, and GS-Mini is a great tool, provided you use the constantly-changing managed speed as a target, not a minimum speed. On the other hand, the A/THR device whereby you can add a burst of thrust by briefly pushing the levers beyond the CLB gate can get you into a lot of trouble, IMHO. Not all airlines permit manual thrust (yet...), and I agree with USMCProbe's sentiments on that issue.
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Old 6th Feb 2013, 18:33
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@Bula

YOUR ARE THE PILOT!

There are several knobs on 321 to make it work like YOU want!
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Old 6th Feb 2013, 20:23
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Hetfield thanks for those inspiring word of wisdom. I'll add it to my golden rules and shall call it the Hetfield guide to Airbus. inspiring stuff.

Thanks for the input guys and Gals. This issue is specifically A321 and specifically V2500 engines. I've got thousands of hours left and right seat in the A320 but only fly the A321 once every other month so haven't really got to know her.

I've flown the A320 into some places which have reputations for nasty shear. Wellington NZ namely being one of my regular haunts. Flap 3, G/S mini are fantastic tools. I generally don't manually up the Vapp if I'm expecting Vapp to be close to vmax - 15 ish as to allow for gusts as it would push the target speed closer to Vmax which is undesirable.

Scotty, after some more research they make the v2500 N1 and N2 spin slightly faster to attain the rated thrust increase hence the lag when higher power is needed. I thought approach thrust was meant to overcome the lag but I doesn't SEEM to be that effective.

Tower Wind 320/10 in PERF page. VAPP in fMGC was Vls + 7 (Manually entered) 400-500' shear. FAC speeds were pretty much as per FMGC. Resulting initial shear was Vls - 2 before I disconnected the ATHR. The ATHR was "lazy" for the entire approach. The low level shear was the last straw. If it doesn't respond then , when will it?

I went Vls + 10 yesterday in a gusty 32 knot crosswind. It seemed to do a better job as it didn't feel like we were slipping on the back of the curve so to speak but the ATHR responsiveness still leaves a lot to be desired for very small vertical rate changes.

Jonty, I was skeptical that a 5 degree change in Flap setting resulting in a lower N1 would have the desired effect with the lag. I'll give it a try. Thanks.

Last edited by Bula; 6th Feb 2013 at 20:32.
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Old 7th Feb 2013, 01:37
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Flown 320 and 321, including CFM & V2500. On the very rare times when I was temporarily not satisfied with autothrust efforts to maintain target speed, just selected the thrust levers very briefly forward of the climb detent. Never had to do more than that. Would hesitate to use same procedure forward of max continuous on single engine, though I cannot recall if a/thr was normally used for one engine inop approach.
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Old 7th Feb 2013, 14:59
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Agree, V2500 A/THR seems lazy. However, the Airbus keeps constant groundspeed on approach (using groundspeed mini), whereas other types keep constant airspeed. So sometimes the Airbus system appears not to be taking enough notice of an airspeed change, or trend, but the groundspeed is usually fine.

I was once told: A/THR keeps speed within 'bug' + or - 5Kts. We pilots tend to want speed at 'bug' + 10, - 0Kts.

I thought the phase advance was very useful. Trouble is, folk would click forward of CLB for much too long and then get into all sorts of trouble. No more than a second then straight back into CLB was all that was ever needed.


U

Last edited by Uplinker; 7th Feb 2013 at 16:05.
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Old 7th Feb 2013, 16:05
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A heavy 321 is a bit of an energy pig. I only flew 321's for about 600 hours. Brand new ones. Where I flew them we had tailwinds on approach almost half the time. But, it was usually smooth.

I would guess that the ultimate test of the autothrust on 320 series aircraft would be a heavy 321 with a tailwind, in gusty conditions. I didn't do that very much. Almost none.

But even on a 319 or 20, they do OK in light to moderate gusty conditions. In Moderate to heavy turbulence, they just can't handle it. They do "do it" better than any Boeing, including the 777. But the organic operator is still expected to be better.

If your airline allows it, practice your skill set. If not, be ready to hit TOGA or the autothrust button, whichever comes first.

I do know that some carriers don't let pilots fly airbus with autothrust off. They are paying you so you have to follow their SOP.

AB autothrust is the best. It is just not perfect yet.
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