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Spoiler-activated Descent Control  
User currently offlineHappy-flier From Canada, joined Dec 1999, 299 posts, RR: 0
Posted (2 years 1 month 3 weeks 2 days 16 hours ago) and read 2925 times:

Sometimes when watching certain aircraft flare - the A343 comes to mind - I wonder why the designers didn't implement a spoiler-activated descent management regimen, as was used in the L1011. That system allowed the aircraft pitch to be maintained, but rate of descent could be varied by "dumping" some lift via simultaneous partial deployment of the spoilers.

Seems like a great feature to have - wonder why the newer FBW airliners don't use it? Or do they?


May the wind be always at your back . . . except during takeoff & landing.
10 replies: All unread, jump to last
 
User currently onlinePihero From France, joined Jan 2005, 4679 posts, RR: 77
Reply 1, posted (2 years 1 month 3 weeks 2 days 15 hours ago) and read 2884 times:
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What you're referring to is the DLC - direct lift control - , one of the features of the Tristar.
This is - very simplistically described - how it works :
From all six spoilers on each wing, spoilers 1 to 4 are extended seven degrees out of the stowed position, when one is in the final approach configuration : flaps at setting 30 or more.
On that final approach, any yoke action - either pilot or A/P induced - will have a primary effect on the 1 to 4 spoilers : a demand for a shallower path would partially retract them and a demand for steeper path will extend them further (to a maximum of 14° ). It's only when the demand exceeds the parameters of the DLC spoilers that the tailplane will intervene.

That system was monstruously complex and it shines on the Lockheed engineers to have pulled the whole system up :
- Mechanically, they had to add DLC to the spoiler roll control, the speedbrake system.... the spoiler mixer was something to behold.
- Electronically, itr was even less simple : consider that as a matter of fact, DLC was controlled by EFCS signals, as a stabilizer out-of-trim signal.
And finally consider the whole tree of conditionals : air/ground; no GoAround.....
The myth is that Douglas, Boeing and Airbus tried to emulate the system, but could not (some patents could have helped !)

Two very beneficial effects arose from the DLC :
1/- Approach stability was phenomenal ( we are talking mid-to-late seventies )
2/- In case of a go-Around, the DLC spoilers would retract with a bang, giving the pilot an immediate 5 tons of lift... and added to the fact that DLC caused a slightly higher thrust setting, hence a quicker engine acceleration, no wonder the L-1011 has had the lowest decision height ever at a few airports I've visited : We tested 12 ft at CDG without a hitch and settled for 15.
Why isn't it used any more ?
Mainly because the newer digital A/Ps were accurate and smooth enough to make a final approach as smooth as the Tristar 35 year's ago... Of course, we'd hit the runway during a go-around at decision height, but it's smooth and quite acceptable... plus, the systems are a lot less complex.

But I still miss the elegance of the Tristar approach.



Contrail designer
User currently offlineHappy-flier From Canada, joined Dec 1999, 299 posts, RR: 0
Reply 2, posted (2 years 1 month 3 weeks 2 days 9 hours ago) and read 2756 times:

Pihero - thanks so much for that phenomenal reply. So much great info in there.

Cheers.



May the wind be always at your back . . . except during takeoff & landing.
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 3, posted (2 years 1 month 3 weeks 2 days 8 hours ago) and read 2741 times:

Quoting Happy-flier (Thread starter):
Seems like a great feature to have - wonder why the newer FBW airliners don't use it? Or do they?

Like pihero said, they don't need it. They do some other very cool stuff with the spoilers, thanks to FBW, but not DLC. Maneuver load alleviation and spoiler gapping are the latest things though. DLC also, by necessity, hikes drag up. That's not a very favorable thing to do these days.

Tom.


User currently offlineMir From United States of America, joined Jan 2004, 21877 posts, RR: 55
Reply 4, posted (2 years 1 month 3 weeks 2 days 7 hours ago) and read 2727 times:

I'd imagine that having the engines spooled up higher on final wasn't very good from a noise abatement standpoint. That's kind of a thing these days.

-Mir



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User currently offlinevikkyvik From United States of America, joined Jul 2003, 10350 posts, RR: 26
Reply 5, posted (2 years 1 month 3 weeks 2 days 6 hours ago) and read 2698 times:

Quoting Pihero (Reply 1):

Was engine thrust at all tied into DLC, or was it a completely separate loop?



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User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 6, posted (2 years 1 month 3 weeks 2 days 2 hours ago) and read 2611 times:

Quoting vikkyvik (Reply 5):
Was engine thrust at all tied into DLC, or was it a completely separate loop?

I dont see why it would be, it would screw up with the principle of operation.



The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
User currently offlinevikkyvik From United States of America, joined Jul 2003, 10350 posts, RR: 26
Reply 7, posted (2 years 1 month 3 weeks 2 days 1 hour ago) and read 2600 times:

Quoting Fabo (Reply 6):
I dont see why it would be, it would screw up with the principle of operation.

I ask because raising and lowering the spoilers will increase/decrease drag. Was just wondering if the system was designed to compensate for that and maintain speed while changing approach angle.

Though I suppose that would just be autothrust.



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User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 8, posted (2 years 1 month 3 weeks 1 day 23 hours ago) and read 2569 times:

The same can be told basically about any airplane, when you extend flaps, drag goes up, you need to add a bit of power. Maybe a bit more in Tristar, but after you are set in approach descent, the same pitch&power principle applies. If any system changed engine settings then, it would mean that handflying would feel very different than any other airplane.


The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
User currently offlinevikkyvik From United States of America, joined Jul 2003, 10350 posts, RR: 26
Reply 9, posted (2 years 1 month 3 weeks 1 day 13 hours ago) and read 2418 times:

Quoting Fabo (Reply 8):
The same can be told basically about any airplane, when you extend flaps, drag goes up, you need to add a bit of power.

I'm aware, thank you.

Quoting Fabo (Reply 8):
If any system changed engine settings then, it would mean that handflying would feel very different than any other airplane.

I'd assume it already does, given that you're adding or subtracting lift rather than pitching up or pitching down....



How can I be an admiral without my cap??!
User currently onlinePihero From France, joined Jan 2005, 4679 posts, RR: 77
Reply 10, posted (2 years 1 month 3 weeks 1 day 10 hours ago) and read 2373 times:
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Quoting Mir (Reply 4):
I'd imagine that having the engines spooled up higher on final wasn't very good from a noise abatement standpoint. That's kind of a thing these days.

I seem to remember that the T* wasn't louder thatn the DC-10, and that was IIRC due to the #2 engine position. But you're right, that sort of set-up would be hard to certify nowadays noise abate-wise.

Quoting vikkyvik (Reply 5):
Was engine thrust at all tied into DLC, or was it a completely separate loop?

There was a component of the AFCS which was called "SCS" for "Speed Control System"' which then had control / monitoring of the ATS aka Auto Throttle System. Those different systems were quite independent of one another (that architecture was unique among all OEMs).

Quoting vikkyvik (Reply 7):
ask because raising and lowering the spoilers will increase/decrease drag. Was just wondering if the system was designed to compensate for that and maintain speed while changing approach angle

As a matter of fact, the Tristar on final was an incredibly steady bird for two reasons :
1/- Inherent inertia of a draggy, heavy body
2/- Energy conservation : If you're high over the glide, you' d push the yoke, causing the DLC spoilers to extend a bit more, causing some drag... in similar conditions, you'd remove some thrust on another aircraft ; here, no need, it seems as if potential energy due to the steeper descent is matched bhy the one caused by the increased drag ; result : one doesn't chase IAS on a Tristar.

[Edited 2012-11-03 15:34:14]


Contrail designer
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