Flaps Skew 737

[Manases Blakemore]

Picturea situation where you have a 1 meter wide board you're pushing down a track that's slightly wider than the board. Your left hand is about a quarter of the way in from the left edge, and your right hand similarly in from the right edge. As long as you push straight down the track, evenly, with both hands, the board slides along and doesn't contact the edge. Now suppose that your right hand pushes harder than your left hand: the board will start to turn, and the edges of the board will contact the edges of the track.

The skew sensor detects that this sort of misalignment is starting to happen, and causes the flap drive to stop. Ideally, this takes place before the flap makes physical contact with any other hardware, so nothing is damaged!

Video of flaps being extended & retracted on the ground. You can see that there are multiple flaps on each wing, and while normally everything moves in synch, you can envision how if one actuator stopped working, the errant flap could get turned pretty far out of alignment, make contact with other hardware, and cause some significant damage. So before this happens, the skew sensor sends the command back to stop everything from moving.

The 757-300 has incorporated a flap skew detection system. Flap skew occurs when either the inboard or outboard edge of the flap moves farther than the rest of the flap. The flap skew detection system monitors, detects, and shuts down the flaps if a skew condition occurs. The heart of this system is the flap slat accessory module (FSAM), taking the place of the flap slat electronic unit (FSEU) number 3. The new LRU was introduced because the existing configuration of the FSEU would not accommodate the addition of the flap skew circuits.

The flap skew sensors are mounted on the trailing edge of the flap jack screws (fig. 4a). The sensor (fig. 4b) is fixed to the flap track, and a target (which contains magnets) is mounted to the jackscrews. As the jackscrew begins to turn, the target also turns. This will pulse the sensor, and a signal (distance traveled) is sent to the FSAM, which compares this signal with the opposite side of the same flap. If the distances traveled by either side of the flap differ by a predetermined amount, the FSAM will shut down the flap system to help eliminate damage to the flap and surrounding structure and the possible loss of a flap in flight. The alternate flap mode will override the normal flap skew detection function, allowing maintenance personnel to move the flaps after maintenance has been performed. Maintenance personnel must verify the flap fault and also ensure that moving the flap will not cause any damage.

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The long posterior flap technique is an established technique for trans-tibial amputation in ischaemic limbs. Despite its success, it has a few drawbacks. It may be time-consuming and requires considerable planning, and at times the dog-ears cannot be avoided. The suture line passes over the distal end of the stump, which is usually a problem during prosthetic use. The skew flap technique retains the advantages of the long posterior flap technique and eliminates the difficulties of prosthetic fitting. The equal skin flaps are skewed so that the flaps become anteromedial and posterolateral, whereas the calf muscle flap remains long underneath the skewed skin flaps. The posterior muscles are brought anteriorly covering the cut ends of the bones and are buried in between the tibia and its anterior periosteum, by suturing their margins with the periosteum. The skew flap procedure was perceived in 1980 and was started at the Artificial Limb Centre, Pune in 1983 by the author. This procedure underwent many changes during the initial 5 years and by the end of April 1992, 85 trans-tibial amputations were performed using this technique. A 9-year follow-up of these patients, who had been using prostheses with ease and without any discomfort or problem, had been exceptionally good. Encouraged by the results, this technique is now being practised as routinely. By March 1998, a total of 125 such trans-tibial amputations had been performed in 119 patients, with excellent results.

Ailerons are powered by hydraulic systems A and/or B. Ifboth hyd should fail, manual reversion is available from both control wheels. Ifthe aileron system jams, the co-pilots wheel can be used to move the spoilers(hydraulically). There are balance tabs and balance panels on both ailerons.

The maximum roll control deflection which the autopilot can achieve is, by design, about 4.5 of aileron movement (a control wheel movement of about 20), depending on the stimulus. The maximum aileron movement which can be achieved by the pilot is about 20, which requires a control wheel input of about 82. (Ref AAIB Bulletin No: 2/97)

Aileron trim moves the neutral position via the feel & centering mechanism. There are two aileron trim switches to prevent spurious electrical signals from applying trim. The fwd switch is for direction, the aft switch is simply an earth return. Use of aileron trim with the autopilot engaged is prohibited because of excessive roll when the a/p is disconnected.

The above series of photographs (737-300) show how the flight spoilers move with various combinations of aileron and speed brake. With speedbrake down, the spoiler simply rises on the down-going wing with aileron. With speedbrake applied, not only do the spoilers on the down-going wing rise but also the spoilers on the up-going wing fall. Notice that even with full speedbrake applied the spoilers still rise on the downgoing wing.

This property of the spoilers on both wings to respond to roll inputs is known as differential spoilers. It only occurs when speedbrake is used which is why the roll rate is increased when speedbrake is used. Boeing recommend that speedbrake is not used below 1000 feet for this reason.

NB In the bottom two photographs the speedbrake lever was only at the flight detent position but because the aircraft was on the ground the ground spoilers deployed. This is why if you have any sort of technical problem that might be due to a faulty air-ground sensor eg QRH "Gear Lever Will Not Move Up After Takeoff" you must not use speedbrake in case the ground spoilers deploy in-flight. You can see from the series of photographs just how much extra drag ground spoilers will give over flight spoilers.

On landing, if armed, all spoilers will deploy when thethrust levers are at idle and any two wheels have spun up or right gear iscompressed. If not armed, the speedbrakes will deploy when reverse thrust is selected.

The SPEED BRAKE TEST buttons are only found on -1/200's and old -300's. They are used in conjunction with the speedbrake lever and antiskid system to illuminate the SPEED BRAKE ARMED and/or SPEED BRAKE DO NOT ARM lights (located next to the fuel gauges).

The above photo (-3/4/500 series) shows how the ground spoilers move more than the flight spoilers and appear to have just two deflection angles. On the 737-NG the story is a little more complex and spoiler movement varies as follows:

The 737 is positively damped in combined lateral-directional oscillations, which in plain English means that if you set up a Dutch Roll the aircraft will gradually stop oscillating. So the yaw damper is not required for dispatch, however it is fitted for passenger comfort. It is poweredby hydraulic system B.

The 1/200 series had a dual yaw damper system because at the design stage, from experience of the 707 and 727, it was expected that the 737 would not be so naturally, positively yaw damped. So two were fitted to allow dispatch in case one failed. As it happened, none were required. The 1/200's have a yaw damper test switch to the right of the indicator. Note that aircraft with the RSEP installed, the yaw damper test switch is inoperative (shown right).

The 737-300 featured a new smaller blade shaped knob butthis was changed to the present round knob following the crash of a US Air 737-400 which overran La Guardia with full left rudder trim set (NTSB/AAR-90/03). It was believed that a jump seat occupant may have inadvertently moved the knob with his foot when it was resting on the aft electronic panel. A raised shield was also fitted around the aft end of this panel to prevent inadvertent movement of the trim controls.

The control column moves the elevators using hyd A and/or B. If both hyd should fail, manual reversion is available from both control columns. If the elevator system jams, the stabilizer (trim) should still be available. There are balance tabs and balance panels on both elevators.

Pitch trim is applied to the stabilizer. Trim can be applied by electric trim switches, autopilot or a manual trim wheel. Electric and autopilot trim may be disengaged by cutout switches on the control stand in the event of a runaway or other malfunction (See here for details of this procedure).

Moving the control column in the opposite direction to electric trim will stop the trim, unless the STAB TRIM switch is set to OVERRIDE. This function could be used to control the pitch of the aircraft with trim say in the event of a jammed elevator.

The trim authority varies according to aircraft series and method of trim. The full range is only available with the manual trim wheel, but if at an extreme setting, electric trim can be used to return to the normal range. There are two electric trim switches on each control column, the right is for the direction and the left is an earth return for protection against spurious electrical signals.

Speed trim is applied to the stabilizer automatically at low speed, low weight, aft C of G and high thrust. Sometimes you may notice that the speed trim is trimming in the opposite direction to you, this is because the speed trim is trying to trim the stabilizer in the direction calculated to provide the pilot with positive speed stability characteristics. The speed trim system adjusts stick force so the pilot must provide significant amount of pull force to reduce airspeed or a significant amount of push force to increase airspeed. Whereas, pilots are typically trying to trim the stick force to zero. Occasionally these may be in opposition.

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