B777 Landing Attitude Modification
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B777 Landing Attitude Modification
Consider the following to be some of the interesting stuff that Boeing just does not tell you about much....
There is tailstrike protection designed into the software but also nosewheel first touchdown protection. While it can vary by model/serial number, LAM will reduce flaperon droop and reflex the ailerons, pushing their neutral position as much as ten degrees up which reduces lift, increases AOA which increases nosewheel clearance.
To quote from the patent(a lot of info not included)....
https://patents.google.com/patent/US5823479
"The probability of aft body contacts during approach and landing involves readjusting a variety of flight parameters to decrease the risk of an aft body contact. These adjustments include increasing landing approach speeds, increasing trailing edge flap deflections at the landing flap detents, and increasing lateral control surface symmetric droop. Vortex generators have been added to airplane wings to address the same problem. All of these solutions succeed in reducing the landing pitch attitude of an airplane, resulting in a greater aft body contact margin. However, at the same time, the solutions reduce the nose gear contact margin. Therefore, while the probability of aft body contacts is decreased, the probability of nose gear first contacts is attendantly increased.
Like aft body contacts, nose gear first contacts can be avoided by adjusting a variety of flight parameters. Decreasing landing approach speeds, decreasing trailing edge flap deflections at the landing flap detents, and decreasing lateral control surface symmetric droop all increase the landing pitch attitude of an airplane. This increase improves nose gear contact margin. The drawback of these various approaches is that increasing pitch attitude attendantly reduces aft body contact margin. The resulting reduction in aft body contact margin consequently increases the probability of aft body contacts.
To overcome the above-mentioned disadvantages, pitch attitude limiting techniques have been proposed. In effect, pitch attitude limiting automatically limits pitch attitudes to a predetermined range. While restrictions on pitch attitude may serve to reduce the probability of nose gear first contacts and aft body contacts, they unduly restrict a pilot's control of the airplane. Restricting a pilot's control of an airplane is undesirable because it violates some fundamental airplane design philosophies, which dictate that a pilot has absolute control of the airplane.
Other proposals to decrease the probability of nose gear first landings without also increasing the probability of aft body contacts have been made. One such proposal, commonly referred to as direct lift control for flight path control, is to provide a closed loop control law which uncouples flight path control from a pitch attitude control. Uncoupling flight path control from attitude control allows changes in flight path angle to be made with little or no change in pitch attitude. Flight path is controlled by modulating wing control surfaces and flaps, not by modifying pitch attitude.
While the direct lift control proposal has some advantages, it has substantial drawbacks. Because the direct lift control technique uncouples flight path control from pitch attitude control, in contrast to the flight control systems of virtually all conventional large commercial transport airplanes, direct lift control would cause airplane handling characteristics unfamiliar to commercial pilots. This proposal would likely require that commercial pilots undergo additional training to learn the different handling characteristics of airplane incorporating direct lift control. Aside from unconventional maneuvering characteristics, direct lift control also poses logistical difficulties. The implementation of direct lift control would necessitate complex system changes to ensure acceptable pilot and flight control system interaction. Furthermore, the use of spoilers, which are often utilized by the direct lift control, could result in unacceptable air frame buffeting, unduly compromising passenger comfort."
".....the invention provides an airplane landing attitude modifier (LAM) that improves nose gear contact margins and/or aft body contact margins. The improved margins result from the automatic, symmetric variation of movable aircraft surfaces and/or high lift surfaces, including, for example, the flaperons. Because the LAM can provide an increased aft body contact margin, the LAM obviates the need for aft body tail skids intended to protect against aft body contacts. Thus, the added weight, degradation in structural clearance, and economic expense associated with aft body tail skids are avoided. Furthermore, because the LAM can improve both the nose gear contact margin and aft body contact margin of an airplane, the limitations associated with the conventional techniques of merely adjusting landing approach speeds, trailing edge flap deflections at the landing flap detents, and lateral control surface symmetric droop are avoided. By allowing improved contact margins at both extreme ends of an airplane's landing pitch attitude envelope, the LAM overcomes the limitations of these adjustments which merely improve aft body contact margin at the expense of the nose gear contact margin or vice versa. Because the LAM does not artificially limit the available pitch attitude for an airplane, in contrast to some conventional pitch limiting methods, the absolute control of the airplane is retained by the pilot.
The LAM 10 positions the flaperons of an airplane to improve the nose gear contact margin and the aft body contact margin during an airplane's landing. The LAM 10 symmetrically adjusts the flaperon droop from the nominal position in response to the difference between an airplane's current approach condition and reference approach condition. The adjustment provides a decreased pitch attitude variation for the airplane's landings. Although the preferred embodiment causes adjustment of the flaperons of an airplane, it is to be understood that the LAM 10 could also be applied to symmetrically adjust other lift generating movable surfaces or combinations of lift generating movable surfaces on an airplane as well."
There is tailstrike protection designed into the software but also nosewheel first touchdown protection. While it can vary by model/serial number, LAM will reduce flaperon droop and reflex the ailerons, pushing their neutral position as much as ten degrees up which reduces lift, increases AOA which increases nosewheel clearance.
To quote from the patent(a lot of info not included)....
https://patents.google.com/patent/US5823479
"The probability of aft body contacts during approach and landing involves readjusting a variety of flight parameters to decrease the risk of an aft body contact. These adjustments include increasing landing approach speeds, increasing trailing edge flap deflections at the landing flap detents, and increasing lateral control surface symmetric droop. Vortex generators have been added to airplane wings to address the same problem. All of these solutions succeed in reducing the landing pitch attitude of an airplane, resulting in a greater aft body contact margin. However, at the same time, the solutions reduce the nose gear contact margin. Therefore, while the probability of aft body contacts is decreased, the probability of nose gear first contacts is attendantly increased.
Like aft body contacts, nose gear first contacts can be avoided by adjusting a variety of flight parameters. Decreasing landing approach speeds, decreasing trailing edge flap deflections at the landing flap detents, and decreasing lateral control surface symmetric droop all increase the landing pitch attitude of an airplane. This increase improves nose gear contact margin. The drawback of these various approaches is that increasing pitch attitude attendantly reduces aft body contact margin. The resulting reduction in aft body contact margin consequently increases the probability of aft body contacts.
To overcome the above-mentioned disadvantages, pitch attitude limiting techniques have been proposed. In effect, pitch attitude limiting automatically limits pitch attitudes to a predetermined range. While restrictions on pitch attitude may serve to reduce the probability of nose gear first contacts and aft body contacts, they unduly restrict a pilot's control of the airplane. Restricting a pilot's control of an airplane is undesirable because it violates some fundamental airplane design philosophies, which dictate that a pilot has absolute control of the airplane.
Other proposals to decrease the probability of nose gear first landings without also increasing the probability of aft body contacts have been made. One such proposal, commonly referred to as direct lift control for flight path control, is to provide a closed loop control law which uncouples flight path control from a pitch attitude control. Uncoupling flight path control from attitude control allows changes in flight path angle to be made with little or no change in pitch attitude. Flight path is controlled by modulating wing control surfaces and flaps, not by modifying pitch attitude.
While the direct lift control proposal has some advantages, it has substantial drawbacks. Because the direct lift control technique uncouples flight path control from pitch attitude control, in contrast to the flight control systems of virtually all conventional large commercial transport airplanes, direct lift control would cause airplane handling characteristics unfamiliar to commercial pilots. This proposal would likely require that commercial pilots undergo additional training to learn the different handling characteristics of airplane incorporating direct lift control. Aside from unconventional maneuvering characteristics, direct lift control also poses logistical difficulties. The implementation of direct lift control would necessitate complex system changes to ensure acceptable pilot and flight control system interaction. Furthermore, the use of spoilers, which are often utilized by the direct lift control, could result in unacceptable air frame buffeting, unduly compromising passenger comfort."
".....the invention provides an airplane landing attitude modifier (LAM) that improves nose gear contact margins and/or aft body contact margins. The improved margins result from the automatic, symmetric variation of movable aircraft surfaces and/or high lift surfaces, including, for example, the flaperons. Because the LAM can provide an increased aft body contact margin, the LAM obviates the need for aft body tail skids intended to protect against aft body contacts. Thus, the added weight, degradation in structural clearance, and economic expense associated with aft body tail skids are avoided. Furthermore, because the LAM can improve both the nose gear contact margin and aft body contact margin of an airplane, the limitations associated with the conventional techniques of merely adjusting landing approach speeds, trailing edge flap deflections at the landing flap detents, and lateral control surface symmetric droop are avoided. By allowing improved contact margins at both extreme ends of an airplane's landing pitch attitude envelope, the LAM overcomes the limitations of these adjustments which merely improve aft body contact margin at the expense of the nose gear contact margin or vice versa. Because the LAM does not artificially limit the available pitch attitude for an airplane, in contrast to some conventional pitch limiting methods, the absolute control of the airplane is retained by the pilot.
The LAM 10 positions the flaperons of an airplane to improve the nose gear contact margin and the aft body contact margin during an airplane's landing. The LAM 10 symmetrically adjusts the flaperon droop from the nominal position in response to the difference between an airplane's current approach condition and reference approach condition. The adjustment provides a decreased pitch attitude variation for the airplane's landings. Although the preferred embodiment causes adjustment of the flaperons of an airplane, it is to be understood that the LAM 10 could also be applied to symmetrically adjust other lift generating movable surfaces or combinations of lift generating movable surfaces on an airplane as well."
Last edited by JammedStab; 4th Apr 2018 at 09:23.
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I can't be sure but it talks about the tailskid being removed...which is the case for the newer-build aircraft and also has this on the right side...."1998-10-20
US5823479A Grant", "Priority date 1996-05-20".
US5823479A Grant", "Priority date 1996-05-20".
Jammed- Yes, tail strike software has been in for a few years, hence deletion of tail-skid, but I wasn't aware of the nose landing mod.
Stilton- sounds a neat trick on a non-FBW aircraft.
Stilton- sounds a neat trick on a non-FBW aircraft.
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LAM features only in normal flight control mode of b777, not in secondary or DIRECT.
I am also assuming it is the reason why there is a limitation/recommendation (in FCTM) of VREF + 20 knots max speed correction in normal circumstances for landing. Wish BOEING was more detailed on these subjects.
I am also assuming it is the reason why there is a limitation/recommendation (in FCTM) of VREF + 20 knots max speed correction in normal circumstances for landing. Wish BOEING was more detailed on these subjects.
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"While the direct lift control proposal has some advantages, it has substantial drawbacks. Because the direct lift control technique uncouples flight path control from pitch attitude control, in contrast to the flight control systems of virtually all conventional large commercial transport airplanes, direct lift control would cause airplane handling characteristics unfamiliar to commercial pilots. This proposal would likely require that commercial pilots undergo additional training to learn the different handling characteristics of airplane incorporating direct lift control. Aside from unconventional maneuvering characteristics, direct lift control also poses logistical difficulties. The implementation of direct lift control would necessitate complex system changes to ensure acceptable pilot and flight control system interaction. Furthermore, the use of spoilers, which are often utilized by the direct lift control, could result in unacceptable air frame buffeting, unduly compromising passenger comfort."
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"Like aft body contacts, nose gear first contacts can be avoided by adjusting a variety of flight parameters. Decreasing landing approach speeds, decreasing trailing edge flap deflections at the landing flap detents, and decreasing lateral control surface symmetric droop all increase the landing pitch attitude of an airplane. This increase improves nose gear contact margin. The drawback of these various approaches is that increasing pitch attitude attendantly reduces aft body contact margin. The resulting reduction in aft body contact margin consequently increases the probability of aft body contacts."
I suspect that the reason for a maximum of Vref+20 is for more than one reason which includes nosegear contact margin.
According to the patent.....
"While the direct lift control proposal has some advantages, it has substantial drawbacks. Because the direct lift control technique uncouples flight path control from pitch attitude control, in contrast to the flight control systems of virtually all conventional large commercial transport airplanes, direct lift control would cause airplane handling characteristics unfamiliar to commercial pilots. This proposal would likely require that commercial pilots undergo additional training to learn the different handling characteristics of airplane incorporating direct lift control. Aside from unconventional maneuvering characteristics, direct lift control also poses logistical difficulties. The implementation of direct lift control would necessitate complex system changes to ensure acceptable pilot and flight control system interaction. Furthermore, the use of spoilers, which are often utilized by the direct lift control, could result in unacceptable air frame buffeting, unduly compromising passenger comfort."
"While the direct lift control proposal has some advantages, it has substantial drawbacks. Because the direct lift control technique uncouples flight path control from pitch attitude control, in contrast to the flight control systems of virtually all conventional large commercial transport airplanes, direct lift control would cause airplane handling characteristics unfamiliar to commercial pilots. This proposal would likely require that commercial pilots undergo additional training to learn the different handling characteristics of airplane incorporating direct lift control. Aside from unconventional maneuvering characteristics, direct lift control also poses logistical difficulties. The implementation of direct lift control would necessitate complex system changes to ensure acceptable pilot and flight control system interaction. Furthermore, the use of spoilers, which are often utilized by the direct lift control, could result in unacceptable air frame buffeting, unduly compromising passenger comfort."
I also read this bit:
The improved margins result from the automatic, symmetric variation of movable aircraft surfaces and/or high lift surfaces, including, for example, the flaperons.
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I assume that this sort of thing is used for the 787 as well. The patents seem to PROBABLY provide answers some of my questions which were not answered in the pilot manuals. The answers are....yes there is tailstrike protection on takeoff even once the aircraft is airborne and yes there is tailstike protection on landing.
Keep in mind that I didn't see the 777 specifically mentioned in Boeing's patent below.
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https://patents.google.com/patent/US6422517B1/en
“Aircraft can achieve high angles of incidence relative to a runway during both takeoff and landing segments of flight. If the angle becomes large enough while the aircraft is close to the ground, the aft or tail portion of the craft may contact the runway surface. Such contact is sometimes referred to as a tailstrike and is generally sought to be avoided. For this reason and others, manufacturers recommend pitch rates and speeds at which takeoff and landing maneuvers are to be performed. In practice, however, variations in both can be expected due to differing pilot techniques and weather conditions. In some instances, takeoff and landing speeds are increased to provide additional aft body margin and thus reduce the probability of tail contact in the event of a large variation in airspeed or pitch rate. Increasing scheduled takeoff or landing speeds is not an optimal arrangement, since it introduces a performance penalty.
Others have sought to prevent tailstrikes by regulating aircraft incidence (angle of attack) by modifying the aircraft's commanded rotation rate. See for example U.S. Pat. No. 5,527,002 in which a percentage of commanded rotation rate is used to avoid a tailstrike. In doing so, the '002 invention does not consider several key parameters that can affect the probability of aft body contact. It is the understanding of the inventors herein that the rate at which the aft body approaches the runway is a function of both the rotation rate of the aircraft and the movement of the center of rotation relative to the runway.
Only when the majority of the weight of the aircraft is on the gear and the aircraft is rotating only about the landing gear, is the motion of the aft body toward the runway almost purely a function of rotation rate. As the wing begins to generate lift and the aircraft begins to climb away from the runway, the motion of the aft body toward the runways becomes a function of the motion of the rotation rate of the aircraft and the motion of the center of rotation relative to the runway. During this period, the center of rotation moves from the landing gear to the center of gravity of the airplane. In addition to this movement, the center of gravity of the airplane begins to move away from the runway as it lifts off. It is during this segment of the rotation, just at or just after liftoff, that many takeoff tailstrikes can occur. By ignoring the motion of the center of rotation, inventions based on pitch and pitch rate alone limit the performance of the aircraft in some situations and provide only limited protection in others.”
“The present invention is a system for preventing tailstrikes of aircraft during takeoff and landing maneuvers. The system accounts for the various rotations and movements of the rotation centers by considering both the height of the aircraft aft body relative to the runway and the rate at which the aft body is actually approaching the runway (i.e., the “tail closure rate”). If the tail closure rate or tail height exceeds an expected boundary, the excess closure rate or excess tail height is used to develop a nose-down pitch command which is then summed onto the normal control law pitch command to help bring the closure rate and tail height to an acceptable value. If the closure rate and tail height stay within the boundary, it is not necessary for the system to alter the pitch command. By looking directly at aft body height and closure rate, all factors contributing to tailstrikes are inherently captured. Further, by predicting when tailstrikes are probable and only intervening in those cases, the current invention does not interfere with normal piloting techniques.”
“The present invention is an improvement to an aircraft flight control system that reduces the likelihood of aircraft tailstrikes by considering such characteristics as tailskid height and tailskid rate during takeoffs and landings. The flight control system includes a pitch command provided to a pitch control device for altering the aircraft's pitch attitude. The improvement is a system of altering the pitch command to avoid an aircraft tailstrike. The improvement includes determining a current tailskid closure rate and a current tail height; comparing the current tailskid closure rate with a threshold closure rate to determine an excess closure rate amount; and adding an incremental nose-down pitch command with the pitch command to avoid a potential aircraft tailstrike. The threshold closure rate is dependent upon the current tailskid height. The incremental nose-down pitch command is calculated as a function of the excess closure rate amount.”
“….the present invention uses the current aircraft tail height and closure rate during takeoff and landing to determine whether pitch attitude intervention is necessary to avoid a tailstrike. By using real-time measurements of height and closure rate, the present invention inherently accounts for the various rotations and movements of the rotation centers. If the tail closure rate exceeds an expected boundary, the excess closure rate is used to develop a nose-down pitch attitude command to help bring the closure rate and height to an acceptable value. By predicting when tailstrikes are probable and only intervening in those cases, the current invention does not interfere with normal piloting techniques unless it is absolutely necessary. In addition, even if the normal pitch command is zero, the present invention will still operate to avoid a tailstrike if need be.”
“Aircraft can achieve high angles of incidence relative to a runway during both takeoff and landing segments of flight. If the angle becomes large enough while the aircraft is close to the ground, the aft or tail portion of the craft may contact the runway surface. Such contact is sometimes referred to as a tailstrike and is generally sought to be avoided. For this reason and others, manufacturers recommend pitch rates and speeds at which takeoff and landing maneuvers are to be performed. In practice, however, variations in both can be expected due to differing pilot techniques and weather conditions. In some instances, takeoff and landing speeds are increased to provide additional aft body margin and thus reduce the probability of tail contact in the event of a large variation in airspeed or pitch rate. Increasing scheduled takeoff or landing speeds is not an optimal arrangement, since it introduces a performance penalty.
Others have sought to prevent tailstrikes by regulating aircraft incidence (angle of attack) by modifying the aircraft's commanded rotation rate. See for example U.S. Pat. No. 5,527,002 in which a percentage of commanded rotation rate is used to avoid a tailstrike. In doing so, the '002 invention does not consider several key parameters that can affect the probability of aft body contact. It is the understanding of the inventors herein that the rate at which the aft body approaches the runway is a function of both the rotation rate of the aircraft and the movement of the center of rotation relative to the runway.
Only when the majority of the weight of the aircraft is on the gear and the aircraft is rotating only about the landing gear, is the motion of the aft body toward the runway almost purely a function of rotation rate. As the wing begins to generate lift and the aircraft begins to climb away from the runway, the motion of the aft body toward the runways becomes a function of the motion of the rotation rate of the aircraft and the motion of the center of rotation relative to the runway. During this period, the center of rotation moves from the landing gear to the center of gravity of the airplane. In addition to this movement, the center of gravity of the airplane begins to move away from the runway as it lifts off. It is during this segment of the rotation, just at or just after liftoff, that many takeoff tailstrikes can occur. By ignoring the motion of the center of rotation, inventions based on pitch and pitch rate alone limit the performance of the aircraft in some situations and provide only limited protection in others.”
“The present invention is a system for preventing tailstrikes of aircraft during takeoff and landing maneuvers. The system accounts for the various rotations and movements of the rotation centers by considering both the height of the aircraft aft body relative to the runway and the rate at which the aft body is actually approaching the runway (i.e., the “tail closure rate”). If the tail closure rate or tail height exceeds an expected boundary, the excess closure rate or excess tail height is used to develop a nose-down pitch command which is then summed onto the normal control law pitch command to help bring the closure rate and tail height to an acceptable value. If the closure rate and tail height stay within the boundary, it is not necessary for the system to alter the pitch command. By looking directly at aft body height and closure rate, all factors contributing to tailstrikes are inherently captured. Further, by predicting when tailstrikes are probable and only intervening in those cases, the current invention does not interfere with normal piloting techniques.”
“The present invention is an improvement to an aircraft flight control system that reduces the likelihood of aircraft tailstrikes by considering such characteristics as tailskid height and tailskid rate during takeoffs and landings. The flight control system includes a pitch command provided to a pitch control device for altering the aircraft's pitch attitude. The improvement is a system of altering the pitch command to avoid an aircraft tailstrike. The improvement includes determining a current tailskid closure rate and a current tail height; comparing the current tailskid closure rate with a threshold closure rate to determine an excess closure rate amount; and adding an incremental nose-down pitch command with the pitch command to avoid a potential aircraft tailstrike. The threshold closure rate is dependent upon the current tailskid height. The incremental nose-down pitch command is calculated as a function of the excess closure rate amount.”
“….the present invention uses the current aircraft tail height and closure rate during takeoff and landing to determine whether pitch attitude intervention is necessary to avoid a tailstrike. By using real-time measurements of height and closure rate, the present invention inherently accounts for the various rotations and movements of the rotation centers. If the tail closure rate exceeds an expected boundary, the excess closure rate is used to develop a nose-down pitch attitude command to help bring the closure rate and height to an acceptable value. By predicting when tailstrikes are probable and only intervening in those cases, the current invention does not interfere with normal piloting techniques unless it is absolutely necessary. In addition, even if the normal pitch command is zero, the present invention will still operate to avoid a tailstrike if need be.”
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As you mentioned Sir,-
AMM extract below for the same
The landing attitude modification (LAM) logic decreases the flaperon droop when the airplane is in an overspeed approach with flaps at the 25-unit or 30-unit position. The PFC calculates the reduction of flaperon droop proportionally to the overspeed increment. Full flaperon droop removal occurs when the airspeed is 20 knots more than the approach landing speed shown in the airplane flight manual.
When the LAM logic is active, it reduces wing lift and causes an increase in angle-of-attack. This increases the nose gear ground clearance.
The last line is in sync with what you mentioned.
AMM extract below for the same
The landing attitude modification (LAM) logic decreases the flaperon droop when the airplane is in an overspeed approach with flaps at the 25-unit or 30-unit position. The PFC calculates the reduction of flaperon droop proportionally to the overspeed increment. Full flaperon droop removal occurs when the airspeed is 20 knots more than the approach landing speed shown in the airplane flight manual.
When the LAM logic is active, it reduces wing lift and causes an increase in angle-of-attack. This increases the nose gear ground clearance.
The last line is in sync with what you mentioned.
