OF FLYING QUALITIES AND GUIDANCE DISPLAYS FOR AN
ADVANCED TILT-WING STOL TRANSPORT AIRCRAFT IN FINAL
APPROACH AND LANDING
Chad R. Frost
James A. Franklin
National Aeronautics and Space Administration
Gordon H. Hardy
Northrop Grumman Information Technology
Ames Research Center
Moffett Field, California
Abstract
A piloted simulation was performed on the Vertical
Motion Simulator at NASA Ames Research Center to
evaluate flying qualities of a tilt-wing Short Take-Off
and Landing (STOL) transport aircraft during final
approach and landing. The experiment was conducted
to assess the designs handling qualities, and to evaluate
the use of flightpath-centered guidance for the precision
approach and landing tasks required to perform STOL
operations in instrument meteorological conditions,
turbulence, and wind. Pilots rated the handling
qualities to be satisfactory for all operations evaluated
except those encountering extreme crosswinds and
severe windshear; even in these difficult meteorological
conditions, adequate handling qualities were
maintained. The advanced flight control laws and
guidance displays provided consistent performance and
precision landings.
Introduction
The STOL aircraft configuration investigated in this
experiment was the Advanced Theater Transport (ATT)
concept developed by the Boeing Company-Phantom
Works
1
. Critical aspects of the design of this aircraft
concern flying qualities and flight control requirements
for a large transport aircraft to perform STOL
operations in demanding weather conditions. Existing
military and civil guidance for control system design is
insufficient for these operations, particularly for a
configuration with reduced static stability and advanced
control augmentation. Most information that exists
comes from STOL flight and simulation experience 30
or more years in the past that relates to aircraft
configurations with conventional aerodynamic surfaces,
mechanical controls, simple rate damper type stability
augmentation systems, and basic instrument displays
2,3
.
Modern designs make use of highly augmented digital
fly-by-wire controls and sophisticated displays, and the
basic aerodynamic configurations tend to have relaxed
static stability with minimal or no conventional tail
surfaces.
While the overall objective of this simulation
experiment was to determine handling qualities,
performance and flight control requirements for a
variety of aircraft and control system configurations,
the data unique to the ATT design is proprietary to
Boeing. This paper addresses the topic of what levels
of handling qualities and approach and landing
performance were attained from the still-evolving ATT
configuration.
STOL Transport Aircraft Concept
The ATT configuration shown in Figure 1 features a
tilt-wing, with a horizontal stabilizer of reduced size
compared to conventional transport aircraft, and
without a vertical tail. It is powered by four cross-
shafted turboprop engines driving eight-bladed
propellers having variable collective and cyclic pitch.
The wing planform includes forward sweep of the
outboard wing sections, and large trailing edge flaps
capable of very large deflection angles. The aircraft is
sized to operate with payloads of up to 80,000 lb from
an austere airfield of 750 1,500 ft in length. The
design STOL approach and landing speed is between 65
and 75 knots.
With the wing tilted at the approach configuration
and flaps fully deflected, the aircraft operates on the
backside of the power-required curve, exemplified by
Figure 1. Boeing Phantom Works Advanced Theater
Transport
2002 Biennial International Powered Lift Conference and Exhibit
5-7 November 2002, Williamsburg, Virginia
AIAA 2002-6016
This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. American Institute of Aeronautics and Astronautics
2
the trend of decreasing flightpath angle with decreasing
airspeed at constant power. At constant pitch attitude,
flightpath changes can be made with power without
appreciably altering airspeed, which is a characteristic
that is expected to reduce pilot workload in performing
the landing approach. Airspeed is controlled through
attitude change, with a gradient on the order of 1.8
knots per degree of attitude. Powered-lift contributions
are primarily a result of the large flap deflection, not of
wing tilt. Wing tilt may thus be used to change the
attitude of the fuselage for a given flight condition
without altering the basic aircraft performance.
The block diagram in Figure 2 provides an overview
of the flight control system with its significant
elements. It includes (1) commands from the
associated control response types introduced by the
pilot through the inceptors, (2) a regulator that acts on
the response type commands and feedbacks from the
aircrafts sensors to produce commanded accelerations
necessary to achieve the pilots intended maneuver and
to stabilize the aircraft against disturbances, (3) a
nonlinear aerodynamic/propulsion model that produces
estimates of the aircrafts current accelerations, and (4)
the control selector that acts on the difference between
the commanded and estimated accelerations to produce
commands to the control effectors. The feedback of the
aircrafts estimated accelerations acts to cancel (or
deaugment) the characteristics of the basic aircraft,
leaving the aircraft to respond to the commanded
accelerations from the control response types and
regulator. Control effectors include aerodynamic
control surfaces and propulsion controls. The
aerodynamic controls consist of wing tilt, ailerons, fast
acting flaps, and a horizontal stabilizer (used only for
pitch trim). Propulsive controls include propeller
collective and cyclic pitch. Allocation of the control
effectors depends on flight conditions and control
effector inter-axis coupling.
Control augmentation for STOL operation consists
of pitch, roll and yaw stability and command
augmentation systems (SCAS) and a height damper for
flightpath response augmentation. Pilot inputs for pitch
and roll control are made using a center stick. Pitch
control augmentation provides either rate
command/attitude hold or attitude command/attitude
hold response; response type is selectable by the pilot
through a discrete switch on the center stick grip. Roll
control augmentation provides rate command/attitude
hold response. The pedals command sideslip.
Propeller thrust is controlled manually through the
throttle levers. The height damper operates through
propeller collective pitch and engine power to improve
flightpath bandwidth in response to throttle lever inputs.
Figure 3. Control system structure. Nonlinear
Aerodynamic
Propulsion
Model
Response
Type
Command
Aircraft
Sensors
Control
Selector
Pilot's
Input
-
+
Regulator -
+
Commanded
Acceleration
Estimated
Acceleration
Control
Effectors
Figure 2. Flight control system architecture
Displays
A head-up display (HUD) and a head-down primary
flight display (PFD) provided primary flight
information, including aircraft attitude, flightpath angle,
airspeed, rate of change of airspeed, altitude, engine
power, wing tilt, flap angle, heading, sideslip, and
distance from the airfield.

An example is shown for the
head-up display in Figure 3 while the head-down
primary flight display appears in Figure 4. Pursuit
guidance symbology added to the display for precision
approaches is also shown in the two figures. Course
and glideslope guidance were provided in the form of a
leader aircraft that followed the desired flight profile.
Speed guidance was shown by the airspeed error tape
on the left wing of the flightpath symbol that displayed
the airspeed error from the desired airspeed. The
guidance display was flightpath-centered and presented
the pilot with a pursuit tracking task for following the
intended transition and approach to landing
4
. The
pilots task was to control the flightpath symbol to
follow the leader using engine power and bank angle,
and to make airspeed corrections using pitch attitude.
Wind indicator
Fuselage reference line
Leader aircraft
Airspeed rate caret
Glideslope reference line
Airspeed error tape (shown slow)
Flight path vector
Figure 3. Head-up display configuration American Institute of Aeronautics and Astronautics
3
Figure 4. Head-down primary flight display
For the approach and landing task evaluated in this
study, the HUD was the primary display used and since
the display formats are similar in concept, only the
HUD elements will be discussed. The flightpath symbol
was quickened to compensate for lags in aircraft
response, while the airspeed rate caret was quickened to
compensate for lags introduced by filtering to suppress
the effects of turbulence. The drive laws for these
symbols and a discussion of pursuit displays is
described in Ref. 4.
To use the displays for lateral and vertical flight
path control down to decision height, the pilot places
the flight path vector on the leader, causing the actual
flight path to converge exponentially on the desired
trajectory. The scaling on the leader (driven by scaled
glideslope and localizer errors) was set to give an
exponential convergence time constant of 15 seconds at
altitudes above 1000 feet, varying linearly to a five
second time constant at altitudes below 100 feet.
Below decision height, the pilot uses the airspeed
error tape and the airspeed rate caret to control airspeed.
The scaling on the airspeed error tape and