MD-80/MD-90 In-Flight Ground Spoiler Lockout Mechanism

The in-flight spoiler lockout mechanism on MD-80 and MD-90 airplanes prevents manual movement of the speed brake lever in flight while flaps are extended. Flight crews, maintenance crews, and engineers for operators of these airplanes can best understand the mechanism by reviewing the regulatory requirements that govern its function as well as its design and operation.

An in-flight ground spoiler lockout mechanism was incorporated on MD-80 and MD-90 airplanes to prohibit inadvertent movement of the speed brake lever during flight with flaps extended. The mechanism does not degrade autospoiler deployment performance during landing rollout. In addition, no single failure in the mechanism prevents the flight crew from manually deploying ground spoilers during landing rollout. The lockout mechanism is operationally transparent to the flight crew until its intended function is required. Understanding the operation of the in-flight ground spoiler lockout mechanism includes knowledge of the following:

1. Regulatory issues.

2. System design.

3. Speed brake lever lockout.

4. Unlocking the mechanism.

Regulatory Issues
The MD-80 certification basis for lift and drag devices is defined by U.S. Federal Aviation Regulation (FAR) 25.697, Amendment 22, which refers only to wing flap controls. Paragraph b of the subpart states that the wing flap control must be designed and located to make inadvertent operation improbable. The spoiler lockout system was not required for U.S. Federal Aviation Administration certification on the MD-80. It was incorporated as baseline after the Canadian Ministry of Transportation (MOT) mandated it for its certification of the MD-80.

The MD-90 certification basis for lift and drag devices, however, was established at a later amendment level than for the MD-80. FAR 25.697, Amendment 46 revised paragraph b to state, "Each lift and drag device control must be designed and located to make inadvertent operation improbable. Lift and drag devices intended for ground operation only must have means to prevent the inadvertent operation of their controls in flight if that operation could be hazardous." This established the requirement for an in-flight spoiler lockout feature. As a result, the MD-90 was certified with the present in-flight spoiler lockout mechanism with weight-on-wheels input. Douglas Products Division had originally defined the MD-90 with the lockout feature as baseline.

System Design
The MD-80 and MD-90 spoiler system consists of three spoiler panels located forward of the flaps on the upper surface of each wing. The two outboard panels on each wing function as flight spoilers that are in phase with aileron input to assist in lateral control of the airplane. The flight spoilers also function symmetrically as airborne speed brakes and as ground spoilers in conjunction with the third inboard panels that deploy only on the ground.

The speed brake lever on the flight deck control pedestal controls symmetrical movement of the flight spoiler panels on both wings during speed brake deployment. Because speed brake deployment with flaps extended is not a desirable aerodynamic configuration for the MD-80 and MD-90 wing, the "Limitations" section of the MD-80 and MD-90 Airplane Flight Manual prohibits this flight control surface configuration.

Speed Brake Lever Lockout
Movement of the speed brake lever moves a closed-loop cable connected to the two outboard flight spoiler control valves on each wing. Movement of the flap/slat handle moves a closed-loop cable connected to the dual-flap control valves in the left main landing gear (MLG) wheel well. Cable sectors are installed in the speed brake cable loop and flap system cable loop under the pedestal. When the flaps are extended eight degrees or greater, a programming cam on the flap sector transmits an input through a torque tube to a lockout arm at the speed brake sector. The lockout arm engages a gate on the speed brake sector and prevents its rotation.

If the flight crew extends the flaps while the speed brakes are deployed, a spoiler/flap extended alert is annunciated, and the flap sector input places the lockout arm in a spring-loaded standby mode. When the flight crew retracts the speed brake lever in response to the alert, the lockout arm engages the speed brake sector and prevents its movement in the extended direction. The lockout remains in effect until an unlocking input is received or the flaps are retracted.

Unlocking the Mechanism
Any one of three independent unlocking inputs associated with landing is required to unlock the speed brake lever for ground spoiler deployment:

If the flight crew makes a sustained effort to manually move the speed brake handle while the system is locked out, the system kinematics allow each of the inputs to overcome the resultant preload in the speed brake sector and unlock the speed brake lever.

The autospoiler actuator provides the primary unlocking input at touchdown as soon as the MLG wheels reach speeds from 390 rpm to 725 rpm. Wheel spin-up transducers located in each of the four axles provide wheel rotation speed data to the ground spoiler control box on the MD-80. On the MD-90, the transducers provide wheel spin data to the antiskid/ autobrake system control unit. Although all four spin-up transducers normally send this information, only two inputs are required for autospoiler operation. The two inputs include one spin-up signal from either the right outboard wheel or left inboard wheel transducers, and one spin-up signal from either the left outboard or right inboard wheel transducers. An increasing-radius cam driven by the autospoiler actuator drives a cam follower arm that is attached to a push-pull cable mounted on the front of the pedestal. Rotation of the cam follower arm unlocks the mechanism by retracting the lockout arm and allowing rotation of the speed brake cable sector. If the speed brake lever is armed, the autospoiler actuator automatically moves it to the ground spoiler position.

An electric solenoid unlocks the speed brake lever within the first two inches of right MLG strut compression at touchdown. This allows the flight crew to manually deploy ground spoilers prior to compression of the nose landing gear (NLG) strut if either spin-up or autospoiler actuator deployment fails to occur at MLG touchdown. The solenoid independently drives the same cam follower arm operated by the autospoiler actuator. This provides an unlocking input through the same push-pull cable to retract the lockout arm and allow rotation of the speed brake cable sector.

Compression of the right MLG strut closes ground spoiler gear interlock relay R2-291 with an input from the D1-57 weight-on-wheel sensor through the B5-80 proximity switch electronic unit (figure 1). Closure of R2-291 allows 28 volts of direct current (V dc) to pass through spoiler control circuit breaker B1-244 to close spoiler lockout -2 relay R2-592. Closure of R2-592 allows 28-V dc power to pass through spoiler lockout circuit breaker B1-1009 to close and latch spoiler lockout -1 relay R2-591. The latching feature ensures that loss of MLG strut compression and the subsequent opening of relays R2-291 and R2-592, which may occur if the airplane bounces after initial touchdown, have no impact on the mechanism. Closure of R2-591 simultaneously supplies 28 V dc through B1-1009 to energize the solenoid, which unlocks the speed brake lever to allow manual movement, as necessary, prior to lowering the nose.

Compression of the NLG strut also results in rotation of the ground shift drum to ground mode. The shift to ground mode provides an unlocking input through a second push-pull cable to retract the lockout arm and allow rotation of the speed brake cable sector. The compressed NLG strut maintains the speed brake lever in an unlocked condition throughout all ground taxi and takeoff operations, regardless of flap deployment.

Compression of the NLG opens ground shutoff relay R2-590, which opens relay R2-591 and de-energizes the solenoid. Since compression of the NLG strut mechanically maintains the speed brake lever in an unlocked condition throughout all ground taxi and takeoff operations, the solenoid unlocking input is no longer required. Therefore, the solenoid is powered only from MLG touchdown to NLG touchdown to maximize its service life.

When the airplane rotates for takeoff, a time delay in ground shutoff relay R2-590 prevents the solenoid from energizing for the first 10 seconds of flight. By then, the airplane is airborne, and the right MLG strut is fully extended. This saves energy and extends the service life of the solenoid, which other-wise would be actuated on NLG strut extension during rotation.

Compression of the NLG strut provides an alternate electrical signal to activate the autospoiler actuator, as well as a backup mechanical unlocking input that deploys if both the autospoiler actuator and solenoid are inoperative.

The MD-80 and MD-90 in-flight spoiler lockout mechanism prevents the undesirable flight control configuration of deployed spoilers in either the speed brake or ground spoiler mode when flaps are extended. This is accomplished through the use of a flap input/spoiler output interlock, and the mechanism is subsequently unlocked by input from any of the three independent sources. While the system was originally required by the Canadian MOT for installation aboard the MD-80, it was later incorporated into the baseline MD-80. The feature is also incorporated on all MD-90 airplanes.

Ken Orpitelli
Design Engineer
MD-80/MD-90 Design Office
Douglas Products Division

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