Keywords

1 Introduction

A side stick or sidestick controller is an aircraft control column (or joystick) that is located on the side console of the pilot, usually on the right hand side, or outboard on a two-seat flightdeck, just as shown in Fig. 1. Side stick is divided into three basic types: mechanical side stick, passive side stick and active side stick.

Fig. 1.
figure 1

Typical side stick layout

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It’s a long history for the development of side stick. The earliest Wright Flyer used a single-axis side stick to control the pitch of the aircraft and White’s most early designs used the same control type which is the mechanical side stick. With the maturation of fly-by-wire technology, side stick began to be used in aircraft flight control widely. The side stick was first applied in military aircraft [1]. In the 1970s, the American F-16 aircraft firstly adopted the fly-by-wire system and side stick as the primary control and bomber B-1B also adopted side stick later. From then on, side stick re-attracted people’s attention. Until 1980s, the first fly-by-wire civil aircraft with side stick as the primary control appeared, which was Airbus A320 series. After that, side stick was used as mail control equipment on the following series of civil aircraft of Airbus. However, Boeing still uses the traditional control column as the primary control equipment from B-707 to B-787 series. The side stick at this stage is called passive stick which lacks of control feedback from the aircraft or the other pilot. To solve this problem, active side stick came up which pilots can feel the force and visual feedback through the active side stick to have better situation awareness.

2 Passive Side Stick

The principal reason for using side stick is ergonomics. Removing the control yokes allows for larger flight displays in the latest cockpits and pilots can move closer to the instrument panels, allowing use of touchscreens. Besides Airbus, other aircraft company are also inclined to apply the side stick as the primary flight control, like Dassault, Embraer, Bell, Sikorsky, Comac, Irkut and Sukhoi.

At present, the side sticks used in civil aircraft such as Airbus series and Gulfstream series are passive side sticks, that is to say, the pilots can not directly feel the force feedback and displacement of surface through the side stick. For the fly by wire aircraft, it has been the mainstream to use side stick instead of traditional control column. Compared with the mechanical manipulation of the traditional control column, side stick has many advantages, however there are some disadvantages. The passive side stick is fixed by spring without information feedback, and the left side stick does not have mechanical or electrical linkage with the right side stick. When the autopilot works, the side stick is locked in neutral position. The displacement of the side stick is input by pilot’s operation and transfer to the angular velocity command, and then compares it with the true angular rate feedback signal which the error is calculated by the control law and finally the operational signal generates. During above operation, the pilot can only feel the gradient force produced by the spring, the inertia and friction produced by the mechanism. The command of side stick (displacement command) is processed by the flight control computer to control the control surface and the pilot does not get any direct feedback through side stick from the control surface.

The advantages of a passive side stick over the traditional center control column are as follows:

  • (1) Side stick is conducive to manipulation of the front controller side.

Stick device unit is more simplified, the side stick device unit is located on the outside console and its length and displacement are much shorter than the traditional center control column, and multiple controllers can be integrated on one side stick. The traditional control column not only widens the distance between the pilot and the dashboard, but also blocks pilot’s normal operation of the front control panel. The removal of the central control column actually precludes operational obstacles in front of the pilot. Coupled with the ability to move the seat forward, it will undoubtedly greatly facilitate pilot’s flight control of the front controller.

  • (2) Side stick can improve the visibility of the front display.

The center control column obstructs the front lower dashboard. This blockage can be worse when the pilot holds and maneuvers the center control column. After removing the central control column and by using side stick, the visibility of the pilots will suddenly clear. The adverse effects on the visibility of the front monitor are eliminated.

  • (3) It can improve the operation performance.

With a side stick control, the pilot can control the flight more precisely and effortlessly and by using side stick there is more space which allows the pilots not only lift their feet on both feet, but also extend the activities of the lower limbs freely to increase flight comfort.

  • (4) Removing the central control column and other devices, in fact, increases the available space at the front of the cockpit.

On the other hand, the replacement of a conventional side stick reduces the amount of space available for the pilot to complete the maneuvering of the aircraft. This provides more possibilities for designing aircraft cockpit with more equipments.

  • (5) It can reduce the control of the system weight, compared with the steering column, the side bar on the cabin space requirements greatly reduced.

  • (6) The weight of primary flight control system is reduced to enhance airline company’s economic income.

  • (7) It can reduce maintenance costs, reduce flight workload, improve aircraft handling quality, facilitate pilots access, facilitate the front controller and so on.

At the same time, passive side sticks also have some disadvantages:

  • (1) Unable to change hands to operate the aircraft.

Since the side stick is located at the outside of the console, the left pilot can only operate the side stick with his left hand and the right pilot can only operate the side stick with his right hand. As a result, it is difficult for pilots to alternately control the side stick with both hands, which is a major disadvantage of the side-stick piloting device.

  • (2) It is difficult for the pilot to perceive the steering effect directly.

With the fly-by-wire aircraft, there is no longer a conventional mechanical connection between the pilot and the controlled surface of the aircraft. When the pilot maneuvered the aircraft, he could no longer directly feel the reaction force after the controlled movement of the aircraft, so it was difficult to correctly sense the effect of the maneuver and make corrections in time.

  • (3) Feedback information is not easy to distinguish.

Compared with the traditional center control column, the displacement of the side stick is usually small. Whether visually or tactile, it is difficult for pilots to feel the effect or effect of their own control action from slight changes in the position of the side stick. When the pilot explicitly discerns the displacement of the side stick, the actual control input may have been excessive.

The pilot’s perception of the change in position of the side stick is usually better than the resolution of the force. Therefore, the use of pure power type side stick will increase the difficulty of pilots perceived control effect. Persistent force in the case of people, the size of the force changes will become less and less sensitive, which is the inherent tactile adaptation. Tactile adaptation also increases the difficulty of pilots correctly sensing maneuvering effects without any other clues.

  • (4) Passive sidebar because there is no linkage, will increase the difficulty of training drivers.

The biggest drawback of the “passive” sides tick now used in civil aircraft is the lack of control feedback from the aircraft or the other pilot.

However the transfer of “active inceptor” technology to the commercial sector from the military is helping to overcome that objection. Active side stick that provide tactile and visual feedback in response to pilot and autopilot commands are used in the F-35 and CH-53K heavy-lift helicopter.

3 Active Side Stick

As early as 10 years ago, many aircraft manufacturers and equipment manufacturers have studied the active side stick and got some progress. Until now, Gulfstream G500, MC-21 and other commercial aircraft have adopted the active side stick and is in the process of airworthiness certification. It is also reported that the Boeing and Airbus are also working on the research of active side stick, and they also consider to adopt active side stick as the primary flight control system on their next generation aircraft.

In addition that the active side stick has the same visual notification about operating authority and aural notification about dual manipulation just like the passive side stick, variable gradient force feedback is provided since the electric motor is used instead of the spring, and the linkage function between left side stick and right side stick is also realized to acknowledge the pilot if there is dual manipulation. Besides, the active side stick also moves along with the control surface when autopilot is engaged. The active side stick can also receive the feedback from the aircraft control surface (such as the aerodynamic load on the rudder surface) which is different from the passive side stick with the spring that can only provide fixed force. It applies the force and displacement feedback to the side stick through actuators, so that the pilot can feel the state of the aircraft more realistically [2].

Compared with passive side stick, there are many advantages for active side stick.

  • The coupling linkage between the left and right side stick makes the relationship between captain and copilot more suitable for human factors, so that the two drivers feel more direct interaction with each other.

  • Variable force gradient gives more flexibility to operate the aircraft.

  • The feeling of change in force allows the pilot to have a direct feeling of envelope limitations (such as soft tilt limits, speed limits, etc.)

  • It can provide soft stop or hard stop.

  • The anti-drive of the autopilot can provide more direct control information for the pilot.

4 Application of Active Side Stick

Active side stick are mostly used in military aircraft, such as F-35 and CH-53K heavy-lift helicopter. Meanwhile, some civil manufacturer also adopt active side stick, such as Gulfstream’s G500 and G600 business jets, and Russian’s MC-21 aircraft. Their active side stick are mainly from several famous suppliers like BAE, UTAS, Sagem and Stirling.

4.1 Sagem’s Active Side Stick

In 2014, Sagem introduced a prototype of its new active side stick unit for civil airplanes and helicopters. This active side stick was demonstrated in a flight simulator featuring a full glass cockpit at NBAA business aviation trade show and exhibition in Orlando, Florida (USA).

This side stick controller features very high dispatch reliability, a robust design that stands up to all types of contingencies and an optimized architecture supporting the real-time adjustment of force feel laws. Based on its stick shaker and stick pusher function and the fine synchronization between all side stick units, this system greatly facilitates pilot/co-pilot coordination and the management of stressful situations.

4.2 BAE’s Active Side Stick

BAE has developed a commercial active side stick with its own investment. The commercial version of the active side stick is dual-duplex, using dissimilar processors so there are no common failure modes between channels. Side stick force and position-sensing is quadruplex redundant to meet certification requirements.

All of the hardware and software are packaged within a single line-replaceable unit that interfaces with the aircraft’s flight control computers, while dedicated electrical links connect the pilot and copilot inceptors. This ensures both side sticks move together in response to both pilot and autopilot commands, providing crew situational awareness equivalent to conventional pilot controls.

BAE’s active side stick has been used on Gulfstream’s business jets G500 and G600, shown in Fig. 2. Besides, Embraer’s KC-390 also adopts BAE’s active side stick.

Fig. 2.
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BAE’s active side stick

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Just like active side stick’s characteristics, BAE’s active side sticks have the characteristics that breakout forces, force displacement gradients and soft stops in each axis.

4.3 UTAS’s Active Side Stick

France-based subsidiary of UTC Aerospace Systems (UTAS) designed a active side stick which can couple the motions of the left-seat and right-seat controllers. The side stick also provides back driven feedback based on the aircraft’s actions, so more pressure is required to make inputs that cause higher manoeuvring loads.

UTAS’s active side stick was adopted on Russia’s MC-21 which was firstly used on a large commercial aircraft.

4.4 Stirling’s Active Side Stick

Stirling has pioneered active control technology since the early 1990s. Stirling’s family of active controls are feature-rich, highly reconfigurable and suitable for single or dual (linked) cockpit configurations. Stirling’s range of active products includes side sticks, throttles, collectives, cyclics and pedals. These controls are extremely compact, fully active and benefit from low acquisition and through-life costs. It has several active side stick and supplies the active sticks and throttles for helicopter flight and the new F-35 pilot training simulators (Table 1).

Table 1. Stirling’s active side sticks
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5 Application Analysis of Active Side Stick

Although there has no civil aircraft equipped with the active side stick that gets airworthiness certification delivered, some commercial aircrafts such as Gulfstream G500 and MC-21 have used the active side stick and are working on the process of airworthiness certification. Besides, Airbus, Boeing and many equipment manufacturers also have done much research on active side stick for the next generation of aircraft. As a new type of aircraft for the next twenty years, the wide body aircraft by COMAC should also consider apply some new technologies such as active side sticks to maintain competitiveness in the future.

However, there are some potential risks for active side stick in airworthiness certification and human factors which need special attention. In order to maintain the competitiveness in the future, the active side stick technology should be studied in detail in the early stage of the aircraft development.

5.1 Control Law Analysis of Active Side Stick

The control law’s difference of the flight control system between active side stick passive side stick is very large. To avoid too much change, the control law of active side stick should develop on the basis of the passive side stick control law. However, the change of manipulation manner with the active side stick is still very obvious.

It’s very significant for the effect on pilots’ control philosophy and the design of new active side sticks.

5.2 Airworthiness Certification Analysis

One of the important issues with active sidebar aircraft is airworthiness certification. There is no active side stick that has got airworthiness certificated on civil aircraft until now. Therefore, it needs more effort to establish a dedicated airworthiness clause by aircraft manufacturers and the airworthiness certification authority. Airworthiness certification should focus on the following questions:

  • Numerical requirements for the stick force and force gradient,

  • Cross-coupling certification between captain and co-pilot side stick;

  • Back-driven certification;

  • Control law certification.

5.3 Human Factors Analysis

  • (1) Force distribution on both sides of the side stick.

Most people are dextromanual, so the force feedback of the side stick should be properly defined to meet both left hand and right hand, then the left and right pilots can comfortably operate the side stick with either left or right hand.

  • (2) Button on the side stick should be rearranged.

The trim button may be added on the active side stick or other buttons be cancelled, so the layout of the buttons on the side stick need rearranged according to the importance.

  • (3) The force gradient need be newly defined.

The manner of safety protection is different between passive side stick and active side stick, and it’s needed to remind pilots the current state of the aircraft through the force change.

  • (4) Force superposition from pitch and roll.

The relationship between the movement of active side stick and surface of the aircraft should be studied, including the force and displacement of active side stick from left, right, forward and afterward movement.

6 Conclusions

In summary, the use of active side stick on civil aircraft is technically feasible, but there is some certain risks in airworthiness certification. In order to maintain the competitiveness in the future, a detailed study of the active side stick technology should be conducted in the early stages of aircraft development. According to the actual situation, even though passive side sticks are adopted, the interface of active side stick should also be reserved to consider the application of active side stick on the improved generation aircraft.

According to the investigation from the pilots, the demand of linkage function of the side sticks is the most urgent, since it can effectively solve the problem of dual input. Therefore, in the application of active side stick, the linkage function of the side stick can be realized first, then the function of variable gradient force with the speed and the stroke, and finally the follow up function of side stick.