Analysis of Flight 1862

Following the accident, the digital flight data recorder of the aircraft was found and analysed [2]. This section provides an analysis of the accident flight based on the data as observed on the DFDR. This includes a description of the aircraft’s perfor-mance and control capabilities following the separation of the right-wing engines.

The results of this analysis are further described in [17, 18].

The Flight 1862 controllability and performance analysis in this Section was used for the validation of the reconstructed aircraft model and the piloted sim-ulator checkout preceding the experimental evaluations in this Action Group (Part IV).

6.2.2.1 Control Capabilities

The aircraft design and certification requirements [3, 4] state that there should be enough controllability to handle a multiple engine failure on one side in order to continue flight. For certification, this requirement has to be demonstrated during flight test up to the so called air minimum control speed or Vmca. This speed is defined as the minimum speed during a failure of the most critical engine at which aircraft control and a fixed heading can be maintained with full rudder and with sufficient lateral control authority to bank 5 degrees into the operating engine(s).

The first sign of an engine failure will be a sudden roll (φ) of the aircraft. If direc-tional control with the rudder pedals is not applied, or with a fixed rudder deflection (δr), thrust asymmetry will cause the aircraft to yaw. Assuming a right multiple en-gine failure for the nominal case with no structural wing damage, the resulting yaw

will create a negative sideslip angle (β) that creates a positive rolling moment to the right ( ¯L_β). Instant control compensation in an engine failure flight condition may consist of applying a rudder pedal input to counteract the yawing moment due to thrust asymmetry ( ¯Nt), a control wheel deflection to counteract the rolling moment due to sideslip ( ¯L_β) and rudder deflection ( ¯L_δr) or applying a thrust reduction on the remaining engines to decrease the yawing moment.

For the case of Flight 1862 (Fig. 6.5), the wing damage caused an additional lift loss (ΔLdamage) and drag increase (ΔDdamage) on the right wing. Because these effects are a function of angle-of-attack, an increase in angle-of-attack will create an additional rolling moment (Δ^¯Ldamage) and yawing moment (Δ^N¯damage) into the direction of the dead engines. This in turn will require more opposite control wheel deflection, especially to counteract bank steepening during manoeuvring. Banking into the dead engines will increase the minimum control speed and therefore reduce the available controllability.

The Flight 1862 accident aircraft was designed to have enough rudder authority to keep the control wheel almost neutral with two engines inoperative on one side.

This flight condition can be maintained up to the remaining engines set at maximum continuous thrust (MCT) corresponding to an engine pressure ratio (EPR) of 1.35 (MCT/EPR 1.35). Note that maximum continuous thrust is defined as the maximum thrust setting at which the engines may be operated for unlimited time. The engine pressure ratio is used here as a measure for the applied power setting and represents the total pressure ratio across the engine (according to the Flight 1862 DFDR, an EPR of about 1.45 was used as the takeoff thrust setting). For the Flight 1862 case, the DFDR indicates that control wheel deflections between 20 to 60 degrees to the left were needed for lateral control and straight flight (Fig. 6(a)). The aerodynamic effects due to the wing damage and degraded effectiveness of the right-wing inboard aileron required larger left wing down control wheel deflections than in the nominal case. The largest deflection of approximately 60 degrees was required for straight and almost level flight. This condition could only be maintained at full rudder pedal and at high thrust (engine #1 set at EPR 1.56 and engine #2 set at EPR 1.45).

As observed on the DFDR data, maximum available rudder was needed during straight flight (constant track angle) to counteract the yawing moment caused by the separated right-wing engines. The traces of the rudder control surface activity as a response to the rudder pedal inputs are shown in Fig. 6(b). In this figure, it can be seen that, between about t=490s and t=790s into the flight, the lower rudder lags the upper rudder when full pedal is applied. The simulation model of the Flight 1862 aircraft, developed during the study in [17, 18], enabled a reconstruction of the DFDR rudder deflections and an analysis of the contribution of their control author-ity to the aircraft’s control capabilities. By applying the DFDR pilot control inputs to the simulation, taking into account the rudder surface hinge moments and partial loss of hydraulic pressure, rudder deflections could be reconstructed subjected to the effects of calculated aerodynamic blowdown and sideslip. As the cause of the lim-ited lower rudder control authority was unknown [2], the lower rudder deflections, as observed in Fig. 6(b), were approximated in the simulation study in [17, 18] by

Fig. 6.5 Flight 1862 aircraft forces and moments for equilibrium flight with separated right-wing engines and right-wing damage

0 200 400 600 800

(a) DFDR control wheel position (maxi-mum deflection +/- 88 deg)

Rudder surface deflection (deg) Upper rudder

Lower rudder

(b) DFDR rudder surface deflections

Fig. 6.6 Flight 1862 Digital Flight Data Recorder (DFDR) control wheel and rudder surface deflections

assuming a reduced lower rudder actuator hinge moment as a failure mode showing a reasonable match with the DFDR rudder deflections.

6.2.2.2 Performance Capabilities

The maximum performance capability indicates the climb capability of an aircraft, for the current condition, that is available with constant airspeed. The actual climb rate of the aircraft may not be equal to the maximum climb capability. In this con-dition the aircraft acceleration is not equal to zero. The maximum performance ca-pability is calculated by differentiation of the aircraft’s specific energy according to the following equation:

dt = rate of change of specific energy (feet/minute)

dH

dt= altitude or climb rate (feet/minute)

V

g= acceleration along the flight path (feet/minute²) g= gravitational acceleration (feet/minute²) V = airspeed along the flight path (feet/minute)

The DFDR indicates that the Flight 1862 controllability and performance con-dition, after separation of the right-wing engines, required engine thrust settings between approximately MCT (EPR 1.3) and overboost thrust (EPR 1.62) (Fig. 6.7).

A high thrust setting (engine #1 set at EPR 1.56 and engine #2 set at EPR 1.45) was needed to sustain almost straight and level flight.

0 200 400 600 800 0.9

1 1.1 1.2 1.3 1.4 1.5 1.6

Time (sec)

Engine pressure ratio (−)

Engine #1 Engine #2

Fig. 6.7 Flight 1862 DFDR engine No. 1 and 2 thrust settings

An energy analysis of the flight using the DFDR data [2] indicated that after the separation of the engines, the aircraft had level flight capability at go-around thrust and at an indicated airspeed (IAS) of approximately 270 knots. Maneuvering ca-pabilities were marginal and resulted in a loss of altitude. A normal load of 1.1g, equivalent to 25 degrees of bank, reduced the maximum climb capability to approx-imately minus 400 feet per minute. At MCT thrust and at an indicated airspeed of approximately 270 knots, maximum climb performance was about minus 350 feet per minute. Below 260 knots, a normal load factor of 1.15g and an angle-of-attack above approximately 8 degrees resulted in significant performance degradation. At an airspeed of 256 knots, a normal load factor of 1.2g (corresponding to about 33 degrees of bank angle) and MCT thrust, maximum climb performance was reduced to minus 2000 feet per minute.