B777-300/ER Semi-Levered Gear
Thread Starter
Join Date: Apr 2008
Location: Test
Age: 35
Posts: 237
Likes: 0
Received 0 Likes
on
0 Posts
B777-300/ER Semi-Levered Gear
I've read the FCOM, not much explanation of the semi-levered gear and how it works, did some googling but no much info as well.
can someone explain as to how a semi-levered gear work to prevent tailstrike?
can someone explain as to how a semi-levered gear work to prevent tailstrike?
Join Date: Jul 2012
Location: Aus
Posts: 154
Likes: 0
Received 0 Likes
on
0 Posts
On the earlier 777 variants, like most other aircraft, when you rotate, all six wheels stay in contact with the ground, and so the aircraft rotates around a point essentially coincident with the middle wheels as the bogie tilts.
On the ER with its semi-levered gear, since the bogie is locked perpendicular to the strut, when you rotate, the aircraft rotates around the aft axle instead - somewhat aft of the original point. If you visualise it (or find some diagrams online), you can tell this has the effect of raising the whole aircraft slightly (as the point where the strut is affixed to the bogie is now lifting off the ground along with the middle axle), affording more ground clearance and a bit more rotation angle capability for the tail.
It doesn't so much prevent a tailstrike directly - but it increases the tailstrike margin.
On the ER with its semi-levered gear, since the bogie is locked perpendicular to the strut, when you rotate, the aircraft rotates around the aft axle instead - somewhat aft of the original point. If you visualise it (or find some diagrams online), you can tell this has the effect of raising the whole aircraft slightly (as the point where the strut is affixed to the bogie is now lifting off the ground along with the middle axle), affording more ground clearance and a bit more rotation angle capability for the tail.
It doesn't so much prevent a tailstrike directly - but it increases the tailstrike margin.
Join Date: Apr 2008
Location: nowhere
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
Just remember that the gear is only locked for takeoff. I am not sure that in the end we have more margin for preventing a tail strike. Depends on how you look at it. For a given weight, I guess so but Boeing also just ups the max allowable weight for a given situation resulting, I assume, in the same margin under limiting conditions.
Subject to confirmation by someone in the know.
How does the aircraft know when to lock the gear? Just like it knows when there has been an engine failure and does certain things for the pilot automatically, it knows that it is taking off and locks the gear. Exact details on how? My only simple answer is... it is smart.
Subject to confirmation by someone in the know.
How does the aircraft know when to lock the gear? Just like it knows when there has been an engine failure and does certain things for the pilot automatically, it knows that it is taking off and locks the gear. Exact details on how? My only simple answer is... it is smart.
Join Date: Jun 2012
Location: india
Age: 39
Posts: 96
Likes: 0
Received 0 Likes
on
0 Posts
Extract of the comments from some test flight of 300ER pilot -
Boeing devised a semi-levered arrangement for the main gear. A hydraulic strut connects the forward part of the wheel truck to the top of the vertical gear strut. On rotation, the diagonal strut locks in place to keep the gear truck perpendicular to the vertical strut. Instead of rotating about the centre axle, the aircraft now rotates about the aft axle, increasing the apparent height of main gear by around 0.3m.
Tailstrike danger
As with the stretched Airbus A340-600, tailstrikes are possible, so the control laws of the fly-by-wire 777-300ER in the pitch axis incorporate a tailstrike protection feature. This is available in the normal mode of the fly-by-wire control system and uses up to 10° nose-down elevator to prevent a tailstrike. Flight-test results show the semi-levered gear and tailstrike protection, along with minor improvements to the brakes, have combined to reduce the -300ER's predicted take-off field length by 305m.
(primarily SLG improves takeoff performance & simultaneously provides added Tail strike protection)
Source unknown -
Boeing 777 semi-levered main gear.
Because the vast majority of the weight of the airplane is borne by the lift of the wings at the time of rotation, the semi-levered gear acts as if it were “pushing” down like a longer gear. This allows a higher pitch attitude for the same tail clearance or more clearance for the same pitch attitude. A hydraulic strut provides the energy to provide this increased takeoff performance. Although designed to increase takeoff capability, the system provides increased tail clearance for the same weight and thrust as nonequipped airplanes.
Timely elevator input can help avoid tail strikes on both takeoff and landing. The tail strike protection command (TSP CMD ) is summed with the pilot’s input to form a total elevator command. The TSP CMD is limited in size to 10 degrees, which allows the pilot to overcome its effects, if desired, by pulling the column farther aft. The size of the TSP CMD is controlled by excessive tailskid rate relative to a nominal threshold of tailskid rate, and by excessive nearness of the skid to the runway, relative to a nearness threshold. Different thresholds are used for takeoff and landing. The TSP CMD is limited to commanding nose down increments only. Tailskid height and rate are computed from radio altimeter signals, pitch attitude, pitch rate, vertical speed, and the length between the radio altimeter location and the tailskid location. A complementary filter is used to provide acceptably smooth rate and height signals. Provisions are included to account for the bending of the forward fuselage when the nose wheel gear lifts off the ground.
To sum it up about SLG - source from boeing -
(1) the ability to tilt the bogie to provide an effectively longer main landing gear during takeoff rotation and liftoff;
(2) the ability to reposition the bogie beam to an appropriate angle for stoWing the landing gear;
(3) the ability to position the bogie beam to an appropriate pitch-up angle in preparation for landing after landing gear deployment, and thereby facilitate an early air-ground sensing upon initial ground contact of the aft bogie Wheels;
(4) the ability to effectively decouple the auxiliary strut during static and ground-roll operations so as to facilitate equal loading of all main gear Wheel and, accordingly, optimum braking ability; (5) the ability to deactivate the functioning of the auxiliary strut that provides the semi-levered action When desired, such as during landing, so that the auxiliary strut acts as a damping device for partially absorbing touchdoWn loads such that the load transmitted to the aircraft is reduced.
Boeing devised a semi-levered arrangement for the main gear. A hydraulic strut connects the forward part of the wheel truck to the top of the vertical gear strut. On rotation, the diagonal strut locks in place to keep the gear truck perpendicular to the vertical strut. Instead of rotating about the centre axle, the aircraft now rotates about the aft axle, increasing the apparent height of main gear by around 0.3m.
Tailstrike danger
As with the stretched Airbus A340-600, tailstrikes are possible, so the control laws of the fly-by-wire 777-300ER in the pitch axis incorporate a tailstrike protection feature. This is available in the normal mode of the fly-by-wire control system and uses up to 10° nose-down elevator to prevent a tailstrike. Flight-test results show the semi-levered gear and tailstrike protection, along with minor improvements to the brakes, have combined to reduce the -300ER's predicted take-off field length by 305m.
(primarily SLG improves takeoff performance & simultaneously provides added Tail strike protection)
Source unknown -
Boeing 777 semi-levered main gear.
Because the vast majority of the weight of the airplane is borne by the lift of the wings at the time of rotation, the semi-levered gear acts as if it were “pushing” down like a longer gear. This allows a higher pitch attitude for the same tail clearance or more clearance for the same pitch attitude. A hydraulic strut provides the energy to provide this increased takeoff performance. Although designed to increase takeoff capability, the system provides increased tail clearance for the same weight and thrust as nonequipped airplanes.
Timely elevator input can help avoid tail strikes on both takeoff and landing. The tail strike protection command (TSP CMD ) is summed with the pilot’s input to form a total elevator command. The TSP CMD is limited in size to 10 degrees, which allows the pilot to overcome its effects, if desired, by pulling the column farther aft. The size of the TSP CMD is controlled by excessive tailskid rate relative to a nominal threshold of tailskid rate, and by excessive nearness of the skid to the runway, relative to a nearness threshold. Different thresholds are used for takeoff and landing. The TSP CMD is limited to commanding nose down increments only. Tailskid height and rate are computed from radio altimeter signals, pitch attitude, pitch rate, vertical speed, and the length between the radio altimeter location and the tailskid location. A complementary filter is used to provide acceptably smooth rate and height signals. Provisions are included to account for the bending of the forward fuselage when the nose wheel gear lifts off the ground.
To sum it up about SLG - source from boeing -
(1) the ability to tilt the bogie to provide an effectively longer main landing gear during takeoff rotation and liftoff;
(2) the ability to reposition the bogie beam to an appropriate angle for stoWing the landing gear;
(3) the ability to position the bogie beam to an appropriate pitch-up angle in preparation for landing after landing gear deployment, and thereby facilitate an early air-ground sensing upon initial ground contact of the aft bogie Wheels;
(4) the ability to effectively decouple the auxiliary strut during static and ground-roll operations so as to facilitate equal loading of all main gear Wheel and, accordingly, optimum braking ability; (5) the ability to deactivate the functioning of the auxiliary strut that provides the semi-levered action When desired, such as during landing, so that the auxiliary strut acts as a damping device for partially absorbing touchdoWn loads such that the load transmitted to the aircraft is reduced.
Good information but still trying to figure out, as JStab has queried, what is the logic for the locking of the semi-levered gear or 'how does it know' when to activate ?
You wouldn't want the bogies unable to 'toe down' on landing, similarly you want them 'locked' for take off.
Suspect the logic for activation / deactivation is a combination of flap position weight on wheels, thrust and airspeed / groundspeed ?
You wouldn't want the bogies unable to 'toe down' on landing, similarly you want them 'locked' for take off.
Suspect the logic for activation / deactivation is a combination of flap position weight on wheels, thrust and airspeed / groundspeed ?
Join Date: Jun 2012
Location: india
Age: 39
Posts: 96
Likes: 0
Received 0 Likes
on
0 Posts
The ground mode sensor comprises a Weight on-Wheels sensor that detects When the main landing gear is bearing Weight, thus indicating that the landing gear is in contact With the ground.
The takeoff mode sensor preferably comprises an engine speed sensor operable to detect When any of the aircraft's engines is operating above a predeter mined speed. This alloWs the control unit to distinguish betWeen a takeoff condition at Which the engines Will be operating at a relatively high speed (e.g., greater than 60 percent fan speed) and a taxi condition or landing rollout at Which the engines typically operate at a relatively loW speed (e.g., less than 60 percent fan speed).
In order for the control unit to provide an immediate unlock signal to the auxiliary strut in the event of a refused takeoff (RTO), the takeoff mode sensor preferably also comprises a thrust lever sensor operable to detect When any of the thrust levers for the aircraft's engines is advanced beyond a predetermined limit, Which indicates a throttled-up condition. Thus, if an RTO occurs during a takeoff roll and the thrust levers are chopped back to idle (i.e., beloW the predetermined limit), the auxiliary strut is immediately unlocked so that the load on the landing gear is evenly distributed to all Wheels for maximum braking efficiency.
The takeoff mode sensor preferably also comprises a ground speed sensor operable to detect When the aircraft is traveling above a predetermined ground speed. In this manner, the control unit is able to distinguish betWeen a takeoff roll and, for instance, a ground test of the engines in Which the engines may be operating at a high speed. Thus, the auxiliary strut can remain unlocked unless the aircraft is actually rolling doWn the runWay at an appreciable speed. Preferably, the control unit is operable to unlock the auxiliary strut upon expiration of a predetermined time period folloWing liftoff of the aircraft from the ground. Liftoff is indicated by a change of state of the signal from the Weight-on-Wheels sensor.
The auxiliary strut has the capability of assuming two different lock-up lengths, one optimiZed for landing and one optimiZed for takeoff. In this manner, a greater amount of bogie beam rotation can be alloWed on landing so that touchdoWn loads are absorbed and damped by the auxiliary strut before the strut locks up.
The takeoff mode sensor preferably comprises an engine speed sensor operable to detect When any of the aircraft's engines is operating above a predeter mined speed. This alloWs the control unit to distinguish betWeen a takeoff condition at Which the engines Will be operating at a relatively high speed (e.g., greater than 60 percent fan speed) and a taxi condition or landing rollout at Which the engines typically operate at a relatively loW speed (e.g., less than 60 percent fan speed).
In order for the control unit to provide an immediate unlock signal to the auxiliary strut in the event of a refused takeoff (RTO), the takeoff mode sensor preferably also comprises a thrust lever sensor operable to detect When any of the thrust levers for the aircraft's engines is advanced beyond a predetermined limit, Which indicates a throttled-up condition. Thus, if an RTO occurs during a takeoff roll and the thrust levers are chopped back to idle (i.e., beloW the predetermined limit), the auxiliary strut is immediately unlocked so that the load on the landing gear is evenly distributed to all Wheels for maximum braking efficiency.
The takeoff mode sensor preferably also comprises a ground speed sensor operable to detect When the aircraft is traveling above a predetermined ground speed. In this manner, the control unit is able to distinguish betWeen a takeoff roll and, for instance, a ground test of the engines in Which the engines may be operating at a high speed. Thus, the auxiliary strut can remain unlocked unless the aircraft is actually rolling doWn the runWay at an appreciable speed. Preferably, the control unit is operable to unlock the auxiliary strut upon expiration of a predetermined time period folloWing liftoff of the aircraft from the ground. Liftoff is indicated by a change of state of the signal from the Weight-on-Wheels sensor.
The auxiliary strut has the capability of assuming two different lock-up lengths, one optimiZed for landing and one optimiZed for takeoff. In this manner, a greater amount of bogie beam rotation can be alloWed on landing so that touchdoWn loads are absorbed and damped by the auxiliary strut before the strut locks up.
Thanks for the detailed clarification Ak.
So. on the ground, while stationary or taxiing the main bogies will be unlocked, once
a predetermined thrust level and groundspeed is attained they will be locked while
accelerating during take off, unlocking in the event of an RTO so when
any braking / deceleration is taking place the bogies are unlocked. if I understand you correctly.
Weight off the wheels, no locking takes place to allow normal 'toe down' touch down ?
A couple more questions, what if the bogies remained locked before and during landing ?
The stresses on the locking mechanism would be very high
What about an immediate go around after a touchdown without the nosewheel
being lowered to the ground ?
It doesn't seem as though the bogies could be locked in this scenario as they never
'untilt' ?
So. on the ground, while stationary or taxiing the main bogies will be unlocked, once
a predetermined thrust level and groundspeed is attained they will be locked while
accelerating during take off, unlocking in the event of an RTO so when
any braking / deceleration is taking place the bogies are unlocked. if I understand you correctly.
Weight off the wheels, no locking takes place to allow normal 'toe down' touch down ?
A couple more questions, what if the bogies remained locked before and during landing ?
The stresses on the locking mechanism would be very high
What about an immediate go around after a touchdown without the nosewheel
being lowered to the ground ?
It doesn't seem as though the bogies could be locked in this scenario as they never
'untilt' ?
Join Date: May 2006
Location: Genf
Age: 58
Posts: 13
Likes: 0
Received 0 Likes
on
0 Posts
The lock is based on speed and flaps position when on ground and most probably other stuff that I forgot. It unlocks when gear up is selected. It does not locks during extension. I do not recall if I saw that into the design documents or if it was in the AMM.
Second, the TSP is not fool proof. If you determined to tail strike you will. If you have access to a full flight you could plot the column input, elevator deflection, tail clearance and in the time history you will see that PFC will modify column response to elevator when getting close to tail strike.
It does not locks on go around.
Second, the TSP is not fool proof. If you determined to tail strike you will. If you have access to a full flight you could plot the column input, elevator deflection, tail clearance and in the time history you will see that PFC will modify column response to elevator when getting close to tail strike.
It does not locks on go around.
