MOP 2: 3-TPA Aircraft Response

The 3-TPA aircraft response data were analyzed by plotting the time history of the load factor (Nz) and bank angle ( ) for the commanded values and the achieved values.

The time history of the altitude and the flight path angle were also plotted to compare with the expected values. From the time history plots, the maximum parameter deviation was noted for the transitory period and for the steady-state period. The transitory period was defined as the time during which the bank angle or load factor was commanded to change with time. The steady-state period was defined as the period of time during which the bank angle and load factor were commanded to remain steady with time. The evaluation criteria were satisfied if the aircraft flew within 0.1 g of the commanded load factor, within 5 of the commanded bank angle, maintained altitude within 100 feet during level maneuvers,

and maintained the flight path angle of 15 within 3 of the predicted value during climbing maneuvers.

The 3-TPA aircraft response results are summarized in Table 4.2, which represents the largest deviation from the desired value of the aircraft state, as observed over the course of four separate flights, for both the transitory period and the steady-state period. Negative values indicate that the aircraft state was less than desired, while positive values indicate that the aircraft state was greater than desired. To compare data between flights and to observe trends, sample time history plots are presented in Figures 4.1 to 4.3, which show data for test point 6. Additional time history plots are presented in Appendix D for test points 7 and 8.

Table 4.2: Maximum Deviation from Desired Aircraft States for 3-TPA Algorithm: 3-TPA Test Dates: 31 Aug-10 Sep 15 Pressure Altitude: 15,000 ft Airspeed: 310 kts Groundspeed

Transitory Period Steady-State Period Test

Point Angle*Bank FactorLoad Altitude

Flight Path Angle

Bank

Angle* FactorLoad Altitude

Flight Path Angle 6 (FWD) N/A -0.30 g N/A N/A +1.3 -0.09 g N/A -4.3

7 (Left) -21.3 -0.29 g -23 ft +0.2 +0.8 -0.05 g +710 ft -5.9

8 (Right) -22.6 -0.42 g +12 ft +0.3 +1.5 -0.05 g +973 ft -7.3

(redindicates values outside expected performance)

*Bank Angles represent magnitude, negative values mean less bank than commanded.

Figure 4.1 shows that the bank angle was maintained within tolerance during the execution of the forward path maneuver (test point 6). Figure 4.2 shows that the load factor was found to lag the commanded value during transitory periods and to overshoot the desired value, but it subsequently stabilized within evaluation criteria during the steady- state periods. The amount of lag in the load factor was a function of aircraft fuel load

ESCAPE Test Point: 6 (FWD Path) Average OAT: -8 C

Test/Virtual Altitude: 15,000/11,500 ft Test Dates: 31 Aug-10 Sep 15 Center of Gravity: 12.5 - 23.8% Test Day Data

Figure 4.1: Bank Angle vs Time for Test Point 6, Forward Path

and center of gravity, where greater fuel weight and forward center of gravity resulted in increased lag. The algorithm did not directly command the flight path angle, but it established the climb angle by commanding a pre-calculated load factor applied over a period of time.

Figure 4.3 shows that although the initial flight path angle was within evaluation criteria, there was variance of approximately 3 between test runs, caused by di↵erences in aircraft weight and center of gravity location. In all cases, the flight path angle decreased throughout the maneuver, and in one out of four test runs, the final flight path angle was less than 12 . Maximum continuous power was applied during the climb, but the aircraft was thrust limited and could not maintain its airspeed. As the airspeed decreased, the VSS attempted to maintain the commanded load factor by increasing the angle of attack. The angle of attack remained below 12 in all cases and did not cause any VSS safety trips. This indicated that the climb angle was not impeded by the loss of airspeed or by

ESCAPE Test Point: 6 (FWD Path) Average OAT: -8 C

Test/Virtual Altitude: 15,000/11,500 ft Test Dates: 31 Aug-10 Sep 15 Center of Gravity: 12.5 - 23.8% Test Day Data

Figure 4.2: Load Factor (Nz) vs Time for Test Point 6, Forward Path

the lift limit, but rather by an inadequate commanded load factor. In fact, the flight path angle was maintained at 15 during the climbing turn maneuvers which will be discussed in Objective 2, because more load factor was being commanded, even though the airspeed was decreasing throughout the maneuver. The inability to maintain a steady climbing flight path angle without artificial limiters in place was a continuing issue. It was a direct result of the open loop nature of the flight path angle control being subjugated to an Nz that was

theoretically designed for a single flight condition. It did not adapt for changes in aircraft parameters such as gross weight and center of gravity.

In the case of test point 6, two of the test runs impacted the simulated terrain as will be discussed in the next section. This indicates that an inability to precisely hold a flight path angle was detrimental to the algorithm’s performance.

The time history plots for test points 7 and 8 (left and right level paths respectively) are presented in Appendix D Figures D.7 through D.12. During execution of the left and right

ESCAPE Test Point: 6 (FWD Path) Average OAT: -8 C

Test/Virtual Altitude: 15,000/11,500 ft Test Dates: 31 Aug-10 Sep 15 Center of Gravity: 12.5 - 23.8% Test Day Data

Figure 4.3: Flight Path Angle ( ) vs Time for Test Point 6, Forward Path

level turn maneuvers, the steady-state bank angle and load factor were maintained within tolerance of the commanded values. During the transitory period, both the load factor and the bank angle lagged the commanded value by an average of 0.25 seconds. This lag, along with small variations in bank angle and load factor, a↵ected the aircraft’s ability to maintain a constant altitude during the turn. The algorithm attempted to maintain a constant altitude by adjusting the commanded load factor based on the bank angle as per:

Nz= 1

cos µ (4.1)

However, the lag caused by aircraft dynamics caused errors in the expected Nzto relation,

and this prevented the aircraft from maintaining level flight. During and shortly after the roll-in, the aircraft descended as much as 80 ft due to insufficient commanded load factor during the transitory period. Once the load factor reached its steady-state value, the aircraft began to climb and the final altitude varied from 100 ft to 973 ft above the starting altitude dependent upon gross weight and center of gravity parameters.

The aircraft’s inability to maintain the desired flight path angle, and the lag between the commanded parameters and the actual parameters prevented the aircraft from flying the maneuvers as planned. Since the algorithm’s collision predictions assumed that the aircraft would fly the maneuvers as planned, the inability to maintain the desired flight parameters could a↵ect the ESCAPE algorithm’s e↵ectiveness against the terrain which will be discussed thoroughly in the next section.