Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
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A Thrust-only Flight-control System as a Backupfor Loss of Primary Flight Controls

KOICHI YAMASAKI*1

YOSHIMASA ICHIHARA*1

HISAKO YASUI*2

SATOSHI HIRANUMA*3

If all control surfaces of an aircraft become inoperative, only the engine thrust can change
the pitching and rolling moment balance to allow the aircraft to fly stably and land safely. In aneffort to actualize such aircraft control while providing as normal an operating feel as possible,
Mitsubishi Heavy Industries, Ltd. (MHI) developed a thrust-only flight-control system consisting ofa flight-control law based entirely on increases/decreases in thrust, an automatic-landing control
law, and a pilot interface. This system was tested and evaluated by a pilot in a domed simulator toensure that the aircraft would be able to fly and land safely, even if all control surfaces becomeinoperative. The results of this test indicate the possibility of improving aircraft safety andincreasing the survivability of crews and passengers in the event of loss of primary flight controls.

|1. IntroductionAn aircraft-control system is designed to lower the probability of losing the aircraft through amultiplex redundant system. Failure of the control system, however, has occurred in past. In 1974,Turkish Airlines Flight 981 experienced a collapse of the aft cargo compartment floor associatedwith losing the cargo door. This damaged the control cables and made the aircraft uncontrollable.
The aircraft crashed a minute later, claiming the lives of all on board. In 1989, United Airlines
Flight 232 lost flight control because the hydraulic pressure for the control surfaces was lost due tothe dispersion of engine parts while in flight. However, an off-duty pilot who had studied thethrottle-only flight-control technique was on board, and with his help, the aircraft reached anairport. The aircraft lost balance just before touchdown, causing the aircraft to break apart andcatch fire when a wing tip crashed into the runway, but more than half the people on board
survived, and the worst-case scenario was avoided. Although these accidents indicate that amalfunction of the surface control system is fatal to flight control, there is a chance of being able tocontrol the aircraft using only the engine throttles. However, such control is a special techniquerequiring exceptional piloting skills, and it takes considerable training time to learn. Even if thecontrol technique is learned during training, if it is applied during an actual emergency situation,
the workload on the pilot at that time will be at a maximum level. Hence, achieving a safe landingby controlling the aircraft as intended using only the engine throttles up until the moment oftouchdown is extremely difficult.
Considering these issues, MHI is aiming to establish “thrust-only flight-control technology,”i.e., the technology to control the attitude of an aircraft by increasing or decreasing the engine
thrust in response to the pilot’s intended operation even after the loss of primary flight controls.1–4
Establishment of this technology will allow a pilot to operate an aircraft to the nearest airport andland on a runway through normal operation of a control terminal, even in an emergency situation
after losing the primary flight controls, and thus enhance overall aircraft safety.
This paper describes the thrust-only flight-control system that was recently developed andthe results of a system evaluation conducted by a pilot in a domed simulator.
*1

Defense Aircraft Division, Aerospace Systems*3Deputy General Manager, Defense Aircraft Division, Aerospace Systems*2Manager, Defense Aircraft Division, Aerospace Systems

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)2|2. Development of a thrust-only flight-control system
This section describes the core factors of the developed thrust-only flight-control system(Figure 1): the thrust-only control law, automatic-landing control law, and pilot interface. Indeveloping the thrust-only flight-control system, we considered a Boeing 747-400 because (1) theengine layout is suitable for thrust-only flight control, (2) it is an in-service aircraft, and (3)engineering data are available from plenty of open references.

Figure 1

2.1(1)

Block diagram of the thrust-only flight-control system

Development of a thrust-only flight-control law and an automatic-landing
control law

Selection of control variables
To control an aircraft using control surfaces, state variables such as normal acceleration,
pitch rate for longitudinal motion, and roll rate or sideslip angle for lateral-directional motion
are commonly defined as the control variables. However, as the control response characteristics
using thrust are quite different from those using control surfaces, these state variables are notnecessarily appropriate as control variables for a thrust-only flight-control law. Accordingly, we
considered control variables appropriate for thrust-only flight control by using a mathematicalmodel of the target aircraft. We selected the control variables from state variables that responddynamically to engine thrust within a bandwidth in which engine can respond. Through thisprocess, the flight-path angle and bank angle were selected as control variables for thelongitudinal motion and lateral-directional motion, respectively. The flight-path angle and bankangle commands are inputted by the pilot control terminal.
(2) Design of the thrust-only flight-control law
We designed the thrust-only flight-control law consisting of a control law to calculate theangular acceleration command and a distributor to provide optimal distribution of themanipulated variables to actualize the angular acceleration. This made it possible to use thethrust of four aircraft engines independently and appropriately in response to flight conditions
and the pilot’s input. The study and development of a thrust-only flight-control law using sucha design has no precedent in the world and is one of the major characteristics of this thrust-only
flight control system. We applied a loop-shaping design procedure consisting of apre-compensator, a post-compensator, and an H-infinity filter when designing the control law.
The feedback variable/signal consisted of the flight-path angle, angle of attack, and pitch rate
for longitudinal motion and the bank angle, roll rate, and yaw rate for lateral-directional motion.
The thrust-only flight-control law generated appropriate throttle commands to each of the fourengines.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
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2.2

Design of the automatic-landing control law
We adopted an automatic-landing control law to mitigate the enhanced workload on thepilot when approaching and landing on the runway. We designed it using a
proportional–integral–derivative (PID) controller as an outer-loop controller, in contrast to thethrust-only flight-control as an inner-loop controller. We defined the feedback
variable/feedback signal as the positional deviation from the glide slope for longitudinal motion
and positional deviation from the localizer for lateral-directional motion. We designed theautomatic landing control law to allow the pilot to override commands even after engaging theautopilot. This was achieved by adding the flight-path angle and bank-angle commands, which
are generated by the automatic-landing control law, to those inputted by the pilot through thecontrol terminal and then inputting them to the thrust-only flight-control system.

Development of the pilot interface(1)

Control terminal
The control terminal is an important factor in the development of a thrust-only
flight-control system because it serves as the interface between the pilot and the thrust-only
flight-control law. Improper settings not only make it difficult for the pilot to operate theaircraft but also have the potential to bring about hazardous oscillatory aircraft motion such as
pilot-induced oscillations because aircraft control based only on increasing or decreasing theengine thrust has a slow response speed compared with normal aircraft control. Taking this into
consideration, we aimed to establish a thrust-only flight-control system that was also applicable
to a broad range of existing and future aircraft. We prepared two types of control terminals, a
column-wheel type and a trim-switch type, based on those in general use in aircraft, andselected the one best suited for thrust-only flight-control after evaluating them in the domed
simulator. Figure 2 (a) shows an outline view of the two control terminals that wereconsidered.
(2) Display
We considered the Boeing 747-400 aircraft when we developed our thrust-only
flight-control system. Based on the instrument panel display of this airplane, with additional
indications thought to be effective for thrust-only flight control, we developed a primary flight
display (PFD) and an engine indication and crew alerting system (EICAS). They had thefollowing new features:
・ The command variable for the longitudinal and lateral-directional motion generated by
the trim switch operation is displayed.
・ The reference velocity vector is displayed as a pilot-support indicator for runway
approach.
・ The target N1 (revolution ratio) is displayed as a pilot-support indicator for aircraft
control by throttle operation.
Figure 2(b) shows the PFD and the EICAS display.

Figure 2

Outline of the pilot interface

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)4|3. Pilot evaluation of the thrust-only flight-control system
A piloted simulation test was conducted using the thrust-only flight-control system, including
the thrust-only flight-control law, automatic-landing control law, and pilot interface. This section
describes the test outline, test case, and one example of the results.

3.1

Test outline

The piloted simulation test was conducted as follows:(1) Date: December 1 and 2, 2010
(2) Place: Domed simulator at MHI(3) Operator: A pilot who works for a Japanese airline(4) Subject of test : An evaluation of various prepared control systems (Table 1).
Details of the test case are given in the following section.
Table 1 Control modes evaluated during the piloted simulation test
Control mode
Conventional
control system

Control systemdeveloped in thisstudy

Conventional

Control by throttle operation

PCA Mode 1

Control by throttle operation (with pilot support indicator)

PCA Mode 2Control through thrust-only flight-control system (column-wheel type)

PCA Mode 3

Control through thrust-only flight-control system (trim-switch type)

PCA Mode 4
PCA Mode 5

3.2

OutlineControl through thrust-only flight-control system (column-wheel type)plus automatic-landing control law (with automatic flare)
Control through thrust-only flight-control system (column-wheel type)plus automatic-landing control law (without automatic flare)

Test caseWe set up the following test case for our piloted simulation test under the assumption that all
the control surfaces were locked at the trim surface deflection for cruise due to the failure of thesurface control system during cruise flight. In this case, the flaps and gear are supposed to beoperable through an emergency alternate mechanism (electrical or mechanical).
(1) Flight-path change during cruise
The controllability for emergency descent and change of direction was evaluated using
each control mode based on the assumption that the aircraft can descend and change direction
only by increasing or decreasing the engine thrust, with all control surfaces locked, to reach toan airport for an emergency landing.
(2) Flight-path maintenance at approach
The controllability for a follow-up to the glide slope and localizer was evaluated using
each control mode based on the assumption that the aircraft can approach the runway only byincreasing or decreasing the engine thrust with all control surfaces locked.
(3) Landing touchdown-point and sink-rate control
The controllability for touchdown-point and sink-rate control were evaluated for eachcontrol mode based on the assumption that the aircraft can land on the runway only byincreasing or decreasing the engine thrust with all control surfaces locked.
(4) Automatic landingThe follow-up performance to the glide slope and localizer, touchdown-point andsink-rate control response, and controllability after the pilot overrides the system wereevaluated for two control modes based on the assumption that the aircraft can approach andland on the runway using the automatic-landing control law with all control surfaces locked.

3.3

Test results and evaluation

Figure 3 shows the flight path for each type of operation during approach, representative of
the aircraft-motion time histories obtained from the piloted simulation test. This figure reveals thatcontrol through the thrust-only flight-control system (PCA Modes 2 and 4) has advanced
controllability, with better follow-up to the glide slope and localizer than a conventional controlmode.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
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Figure 3

Flight path using each control system

Table 2 shows the major pilot comments received during the test. PCA Modes 2 and 3,
which were controlled using the thrust-only flight-control system, had remarkably better
controllability than a conventional control mode and PCA Mode 1, which were controlled throughthrottle operation. PCA Mode2, with a column-wheel control terminal, showed more positiveresults; thus, a column-wheel control terminal was deemed suitable for thrust-only flight control.
For the automatic-landing control law, PCA Mode4, which performed the flare automatically, wasmore desirable compared with PCA Mode 5, which required the pilot to override the system andperform the flare manually.
The test cases demonstrated that the controllability of the thrust-only flight-control system
was remarkably enhanced compared with conventional systems and was effective as an alternate
flight-control means after the loss of primary flight controls.
Table 2 Pilot comments
Control mode

Pilot comments

Conventional
Conventional Unworthy of evaluation. Uncontrollable.
control system
・ I could only operate the throttle to follow the target N1 indication.
・ Follow-up to the glide slope could be done successfully compared with a
PCA Mode 1
conventional control system, but I cannot evaluate in terms of whether toimplement the task.
・ This mode is effective, with much better controllability compared with a
PCA Mode 2
conventional control system and PCA Mode 1.
・ Unique indication of the trim-switch command level is a useful reference.
Control system
・ This mode is effective, with much better controllability compared with adeveloped in PCA Mode 3
conventional control system and PCA Mode 1.
this study
・ This mode needs a little getting used to compared with PCA Mode 2.
PCA Mode 4 The automatic-landing control law is dependable and can be trusted.
PCA Mode 4 which makes the flare maneuver automatically is more
PCA Mode 5
dependable and preferable.
・ It is impossible to carry out landing through throttle operation.
・ The thrust-only flight-control system is very useful and practical because itcan even conduct landing operations.
General・ As long as the aircraft is equipped with this system, I will choose to head toan airport for landing even after the loss of primary flight controls.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)6|4. Conclusion
We developed a thrust-only flight-control system consisting of a thrust-only flight-control
law, an automatic-landing control law, and a pilot interface and conducted a pilot evaluation in ourdomed simulator. We received high acclaim, such as “the developed thrust-only flight-controlsystem is very useful and practical enough for real emergency situations.” If this system can bedemonstrated in actual aerodynamic circumstances, such as during a flight demonstration with asmall experimental airplane or an actual airplane, the system will be considered more practical.
Therefore, we are working on this project continuously.
This study was commissioned by the Advanced Aircraft Technology Development Center of
the Society of Japanese Aerospace Companies (SJAC) as part of the “Assistance Project for
Investigative Study to Enhance the Competitiveness of Aircraft Industries.”

References
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2.
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4.

Shibuya, H. et al., Research and Development for Fault Tolerant Flight Control System - Study of
MuPAL-Alpha's Emergency Guidance and Control using Only Engine Thrust, The 45th Aircraft
Symposium (2007) p.465-469
Yamasaki, K. et al., Development of Thrust-Only Flight Control System as Backup to Loss of Primary
Flight Controls (First Report)，The 48th Aircraft Symposium (2010) p.31-39
Yamasaki, K. et al., Study on Thrust-Only Flight Control System as Backup to Loss of Primary FlightControls, The Society of Japanese Aerospace Companies, Assistance Project for Investigative Study toEnhance the Competitiveness of Aircraft Industries - Report No.2109 (2010)
Yamasaki, K. et al., Study on Thrust-Only Flight Control System as Backup to Loss of Primary FlightControls, The Society of Japanese Aerospace Companies, Assistance Project for Investigative Study toEnhance the Competitiveness of Aircraft Industries - Report No.2202 (2011)