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B-777
- Joined
- Nov 17, 2005
- Posts
- 161
There is something about the L-1011 horizontal stabilizer that makes it unique: In the early design stages, the airlines told Lockheed they wanted the pilots to have more power and control over the pitch system, so that in case of a jet upset, the crew could pull the aircraft out of a dive.
Lockheed designed this extra power into the tail: The L-1011 is the only commercial jet that can take off with full nose down trim. This was actually demonstrated to the airlines during the flight testing phase - with full nose down trim (which is a mis-set trim setting but has happened in the past on other aircraft and caused accidents) the pilot is still able to rotate the aircraft and climb away - it takes a lot of control wheel force to do it but it can be done.
In other aircraft, the control column can be pulled full aft but with full nose down trim the nose won't come up for rotation. For those who don't know: Jet upset was an early event that happened more in the 1950's and early 1960' with the commercial jets than we see today - via rough air, the aircraft would get knocked out of it's stablized cruise condition until it was diving towards the ground out of control - the pilots would attempt to pull back on the wheel to get the nose to come up but most of the jets of that era did not have the power to overcome the excessive speed and thus they could not pull the aircraft out of the dive. Yikes!
When the L-1011 was being designed, the airlines wanted to make sure the L-1011 had enough pitch force to do this should it happen - to this day, a TriStar has never been lost as a result of jet upset and not being able to pull out of the resulting dive.
Now on to the technical description of the L-1011 pitch control system. The L-1011 incorporates an all-flying horizontal stabilizer to control movement in the pitch axis. This "all flying tail" is unique in the commercial aircraft industry (but not general aviation). Pitch control on most airplanes are usually controlled by elevators - on the L-1011 however, the primary part of the tail that controls pitch is the front part called the horizontal stabilizer, the leading edge moves up and down - the elevators are attached to the rear spar of this movable stabilizer piece - the elevators do move, yes, but not under direct pilot control.
The elevators move as a direct result of stabilizer movement only via a physical mechanical link: When the stabilizer moves it's nose downward, the elevators deflect upward to increase the camber of the entire tail - this causes a downward moment and thus pulls the entire rear portion of the aircraft in a downward direction - this causes the nose of the aircraft to pitch up. When the pilot pushes the control wheel forward, this causes the stabilizer nose to move up, causing the elevators to deflect downward, increasing the camber effect to cause a lifting moment and thus pull the tail of the aircraft upward and this in turn causes the nose to pitch down.
The L-1011 has four hydraulic systems - by contrast, the DC-10/MD-11 only have three hydraulic systems. All four of the 1011 hydraulic systems power the horizontal stabilizer, any one of which can operate it. Not all of the flight controls are powered by all four of the hydraulic systems - most are powered by three of the four and only the tail is powered by all four - this is because the pitch system is considered to be the most critical.
Like all widebodied commercial jets, the flight controls are only powered by the hydraulics - there is no manual cable backup system - if you lose all the hydraulics to a given flight control, you lose that flight control.
Note1: Recall the UAL DC-10 flight that crashed at Souix City IA. They lost all three of their hydraulic systems!! They had NO hydraulic control of their flight controls. They used differential thrust to control bank and the electric trim motor to control limited pitch movement. When you realize that they had no hydraulics and no flight control surfaces working, it was absolutely amazing that they got anywhere near an airport runway threshold - but they did - they almost made that landing - if that wing just didn't drop - what an outstanding job they did and talk about flying under pressure!
Note 2: Captain Calvin Dryer took off from SFO in a Pan Am B-747 one sunny day. During their taxi, ATC asked them if they could change departure runways. They said "Sure". They started their take off roll. Suddenly, the end of the runway came up way too soon and Calvin just rotated - early - there was nothing else he could do! No runway remaining to abort, he had some speed but a bit shy of Vr.
The 747 rotated to high - the tail came down and smashed a runway pier which punched thru the cargo area and actually came up thru the pax floor and broke a passenger's leg. The 747 continued to fly and finally got into the air but too slow.
Calvin keep the power on and the jet finally was climbing out but 3 of the 4 hydraulic systems had been lost!! The runway pier severed 3 lines. They were flying on only one system. There was no emergency checklist to cover this condition - Boeing had said the 747 would never loose 3 out of 4 hydraulics. Now, on their on, the crew started to dump their huge fuel load.
News cameras had time to catch the landng for the evening news. The touch down was hard. The flare was insufficient due to only one of the four elevators working. The plane hit hard and bounced back into the air but came down again. Calvin went into reverse thrust but one engine didn't reverse - this was pulling the plane to the right - you could see the split rudder in action - Calvin had full left deflection trying in vain to correct back to center line. Again, having only half of the rudders working, he could not correct and the 747 slid off the right side of the runway.
Finally coming to rest, the doors opened and the passengers started to evacuate. Then as people were jumping out the doors, the plane became imbalanced and suddenly rocked back on it's tail. The slides on the front doors were not long enough to touch the ground - they now hung vertical and people, not knowing this, continued to exit from the front doors!! Many broke leg bones when they hit the ground!
When the 747 switched runways, Calvin forgot to re-calculate the take off performance. Had he done that, he would have found out that the new runway, for his aircraft's weight, was too short to make the take off.
Two things came out of this accident:
(1) The airline manufuacturers, regardless of how many hydraulic systems were on an aircraft, developed emergency checklist procedures to cover the loss of all systems minus one.
(2) Evac slides were mandated to be long enough to touch the ground with any combination of gear collaspe or body angle.
Lockheed designed this extra power into the tail: The L-1011 is the only commercial jet that can take off with full nose down trim. This was actually demonstrated to the airlines during the flight testing phase - with full nose down trim (which is a mis-set trim setting but has happened in the past on other aircraft and caused accidents) the pilot is still able to rotate the aircraft and climb away - it takes a lot of control wheel force to do it but it can be done.
In other aircraft, the control column can be pulled full aft but with full nose down trim the nose won't come up for rotation. For those who don't know: Jet upset was an early event that happened more in the 1950's and early 1960' with the commercial jets than we see today - via rough air, the aircraft would get knocked out of it's stablized cruise condition until it was diving towards the ground out of control - the pilots would attempt to pull back on the wheel to get the nose to come up but most of the jets of that era did not have the power to overcome the excessive speed and thus they could not pull the aircraft out of the dive. Yikes!
When the L-1011 was being designed, the airlines wanted to make sure the L-1011 had enough pitch force to do this should it happen - to this day, a TriStar has never been lost as a result of jet upset and not being able to pull out of the resulting dive.
Now on to the technical description of the L-1011 pitch control system. The L-1011 incorporates an all-flying horizontal stabilizer to control movement in the pitch axis. This "all flying tail" is unique in the commercial aircraft industry (but not general aviation). Pitch control on most airplanes are usually controlled by elevators - on the L-1011 however, the primary part of the tail that controls pitch is the front part called the horizontal stabilizer, the leading edge moves up and down - the elevators are attached to the rear spar of this movable stabilizer piece - the elevators do move, yes, but not under direct pilot control.
The elevators move as a direct result of stabilizer movement only via a physical mechanical link: When the stabilizer moves it's nose downward, the elevators deflect upward to increase the camber of the entire tail - this causes a downward moment and thus pulls the entire rear portion of the aircraft in a downward direction - this causes the nose of the aircraft to pitch up. When the pilot pushes the control wheel forward, this causes the stabilizer nose to move up, causing the elevators to deflect downward, increasing the camber effect to cause a lifting moment and thus pull the tail of the aircraft upward and this in turn causes the nose to pitch down.
The L-1011 has four hydraulic systems - by contrast, the DC-10/MD-11 only have three hydraulic systems. All four of the 1011 hydraulic systems power the horizontal stabilizer, any one of which can operate it. Not all of the flight controls are powered by all four of the hydraulic systems - most are powered by three of the four and only the tail is powered by all four - this is because the pitch system is considered to be the most critical.
Like all widebodied commercial jets, the flight controls are only powered by the hydraulics - there is no manual cable backup system - if you lose all the hydraulics to a given flight control, you lose that flight control.
Note1: Recall the UAL DC-10 flight that crashed at Souix City IA. They lost all three of their hydraulic systems!! They had NO hydraulic control of their flight controls. They used differential thrust to control bank and the electric trim motor to control limited pitch movement. When you realize that they had no hydraulics and no flight control surfaces working, it was absolutely amazing that they got anywhere near an airport runway threshold - but they did - they almost made that landing - if that wing just didn't drop - what an outstanding job they did and talk about flying under pressure!
Note 2: Captain Calvin Dryer took off from SFO in a Pan Am B-747 one sunny day. During their taxi, ATC asked them if they could change departure runways. They said "Sure". They started their take off roll. Suddenly, the end of the runway came up way too soon and Calvin just rotated - early - there was nothing else he could do! No runway remaining to abort, he had some speed but a bit shy of Vr.
The 747 rotated to high - the tail came down and smashed a runway pier which punched thru the cargo area and actually came up thru the pax floor and broke a passenger's leg. The 747 continued to fly and finally got into the air but too slow.
Calvin keep the power on and the jet finally was climbing out but 3 of the 4 hydraulic systems had been lost!! The runway pier severed 3 lines. They were flying on only one system. There was no emergency checklist to cover this condition - Boeing had said the 747 would never loose 3 out of 4 hydraulics. Now, on their on, the crew started to dump their huge fuel load.
News cameras had time to catch the landng for the evening news. The touch down was hard. The flare was insufficient due to only one of the four elevators working. The plane hit hard and bounced back into the air but came down again. Calvin went into reverse thrust but one engine didn't reverse - this was pulling the plane to the right - you could see the split rudder in action - Calvin had full left deflection trying in vain to correct back to center line. Again, having only half of the rudders working, he could not correct and the 747 slid off the right side of the runway.
Finally coming to rest, the doors opened and the passengers started to evacuate. Then as people were jumping out the doors, the plane became imbalanced and suddenly rocked back on it's tail. The slides on the front doors were not long enough to touch the ground - they now hung vertical and people, not knowing this, continued to exit from the front doors!! Many broke leg bones when they hit the ground!
When the 747 switched runways, Calvin forgot to re-calculate the take off performance. Had he done that, he would have found out that the new runway, for his aircraft's weight, was too short to make the take off.
Two things came out of this accident:
(1) The airline manufuacturers, regardless of how many hydraulic systems were on an aircraft, developed emergency checklist procedures to cover the loss of all systems minus one.
(2) Evac slides were mandated to be long enough to touch the ground with any combination of gear collaspe or body angle.
