Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 49
ADVERTISEMENT
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
31 
32 
33 
34 
35 
36 
37 
38 
39 
40 
41 
42 
43 
44 
45 
46 
47 
48 
49 
50 
51 
52 
53 
54 
55 
56 
57 
58 
59 
60 
61 
62 
63 
64 
65 
66 
67 
68 
69 
70 
71 
72 
73 
74 
75 
76 
77 
78 
79 
80 
81 
82 
83 
84 
85 
86 
87 
88 
89 
90 
91 
92 
93 
94 
95 
96 
97 
98 
99 
100 
101 
102 
103 
104 
105 
106 
107 
108 
109 
110 
111 
112 
113 
114 
115 
116 
117 
118 
119 
120 
121 
122 
123 
124 
125 
126 
127 
128 
129 
130 
131 
132 
133 
134 
135 
136 
137 
138 
139 
140 
141 
142 
143 
144 
145 
146 
147 
148 
149 
150 
151 
152 
153 
154 
155 
156 
157 
158 
159 
160 43
backdriven with a force equivalent to about 130 percent of a single functioning
77
PCA.
Calculations showed that at 280 knots (the accident airplane’s airspeed
when the initial descent began), this position would initially have been about
6º nose-down elevator deflection, and the degree of deflection would be
reduced as the airplane’s speed increased above 290 knots. See figure 2 for
78
additional elevator blowdown position information.
•
During the ground tests, the nonfailed elevator surface remained in its
prefailure position unless it received inputs from either control column. A
study of the elevator control system’s force balance and calculations of the
effect of this failure under the conditions of the accident flight indicated that
the nonfailed surface would remain in its prefailure position.
•
During the ground tests, either control column could be used to control the
nonfailed elevator surface and to command the full travel of that surface
available at the existing flight condition. The Safety Board’s study of the
elevator control system indicated that under the accident flight conditions,
inputs from either control column would have resulted in corresponding
movement of the nonfailed elevator surface.
2. A jam of the input linkages or servo valves in two of the three PCAs on the
right elevator surface.
•
During the ground tests, the failed elevator surface was driven to its full
nose-down position and would not respond to nose-up flight control inputs
from either control column. A study of the elevator control system indicated
that if this scenario occurred in flight, it would result in an initial deflection of
the failed surface to a position consistent with a single functioning elevator
operating at 100 percent of its maximum force (as limited by aerodynamic
blowdown forces); the failed elevator surface would resist being backdriven
with a force equivalent to about 130 percent of a single functioning PCA. As
discussed in connection with the previous failure scenario, calculations showed
that, under the conditions of the accident flight, this position would initially
have been about 6º nose-down elevator deflection, and the degree of deflection
would be reduced as the airplane’s speed increased above 290 knots.
•
During the ground tests, the nonfailed elevator surface moved to about
4º nose-down deflection in the same direction as the failed surface. A study of
the elevator control system’s force balance and calculations of the effect of this
failure under the conditions of the accident flight indicated that the nonfailed
79
surface would move to a position corresponding to 30 lbs of feel force.
77
For a detailed explanation of why a failed surface would deflect to this position, see the Aircraft
Performance Group Chairman’s Factual Report and its addendum 1.1.
78
Elevator hinge moment data provided by Boeing were used to estimate the 767 elevator blowdown
positions during the first three failure scenarios. Boeing extrapolated available elevator data based on
Boeing 777 wind tunnel data, which were available for Mach numbers 0.91, 0.94, and 0.96. (As previously
stated, the 767 and 777 have aerodynamically similar horizontal stabilizers and elevators.)
NTSB/AAB-02/01
ADVERTISEMENT
0 votes
Related Articles
Related forms
Related Categories
Parent category: Legal