FlarePath

What happened

NTSB summary, verbatim

The airplane departed in heavy rain and climbed to altitudes where the ambient temperatures were below freezing. During the flight, the crew recognized abnormal turn execution while on autopilot and disconnected the autopilot. Control wheel forces were excessive and aileron response was limited. The crew’s attempts at corrective action, including a roll disconnect to isolate the left and right aileron control circuits, were unsuccessful, and they elected to divert to an alternate airport. While descending, high control wheel force input by the flight crew resulted in the left aileron control returning to normal operation. A few minutes later, the ambient temperature rose to above freezing and an uneventful landing was made. Examination of the control system did not reveal any areas where binding had occurred; however, the events of the flight were consistent with water accumulating in the aileron control system, freezing at higher altitudes, and temporarily binding the aileron control system.

Photographs

1the aircraft, and what the investigators found

The aircraft before the accident

Probable cause

the Board's determination

The malfunction of the airplane’s aileron control system due to ice accumulation.

Occurrence sequence

1 stepsNTSB coding · CICTT taxonomy

1 · Enroute-climb to cruise
Flight control sys malf/fail
defining event

Findings

2 causal · 0 contributing

CAUSE
Aircraft › Aircraft systems › Flight control system › Aileron control system — Malfunction
CAUSE
Environmental issues › Conditions/weather/phenomena › Temp/humidity/pressure › Conducive to structural icing — Effect on equipment

Sequence of events

12 timed eventsfrom the FDR factual report · claude-fable-5-1 (in-session)

24,615 s (preflight)pre-flightp.2
Flight control ground check - ailerons responded normally to control wheel inputs
25,327 sclimbp.2
Ambient temperature fell below 0 C in the climb
25,900 sclimbp.2
Autopilot on: left aileron movement abnormal, limited near neutral and not responding to autopilot inputs; right aileron responding normally; roll disconnect handle IN
26,000 sclimbp.2
Climbing through 26,701 ft at -33.6 C, autopilot on, aileron trim -0.55 deg, left/right aileron 1.06/-0.19 deg, no handwheel force; right aileron frequency reducing, left aileron not following the right (autopilot servo is on the right circuit)
26,135 sclimbp.2
Autopilot disconnected at 28,940 ft, -39.7 C; first left handwheel input 5.96 lb over 2 s moved the ailerons only 0.03-0.04 deg (within breakout 4-7 lb); second right handwheel input of -12.4 lb produced no significant aileron displacement where design would give about -30 deg wheel and -10 deg aileron - indicative of binding
26,137-26,155 sclimbp.3
Four alternating right handwheel inputs of 24.78 to -22.44 lb (expected wheel >60 deg) moved the left aileron only 1.06-1.78 deg and the right -0.12 to -0.34 deg - system binding; 29,167 ft, -40.1 C
26,155-26,245 sclimbp.3
Left handwheel inputs -32.80 to 27.17 lb; left aileron 0.69-1.82 deg, right -0.78 to 0.28 deg - binding; 30,055 ft, -42.4 C; at 26,233 s aileron trim cycled -0.55 to 0.16 to -0.61 to -0.2 deg with negligible aileron response (30,588 ft, -43.9 C)
26,245-26,260 sclimbp.4
Two right handwheel inputs 8.01 to -19.40 lb; left aileron 0.91-1.50 deg, right -0.28 to 0.72 deg - binding; 30,878 ft, -44.7 C
26,266 sclimbp.4
Roll disconnect handle pulled (right and left control wheel positions no longer synchronised; right wheel controls only the right aileron) at 31,095 ft, -45.3 C; right handwheel inputs -15.21 to 14.96 lb to 26,280 s still gave far less than the expected >60 deg wheel - binding on the right circuit too; from 26,280 s roll controlled from the left side
26,280-26,300 sclimbp.5
Left handwheel inputs 21.33 to -13.93 lb moved the left aileron 2.22 to -0.59 deg - binding/increased friction on the left circuit; 31,481 ft, -46.3 C
26,471 scruisep.5
Last right control wheel input: wheel to 11 deg over 10 s moved the right aileron 0.56 deg; 34,333 ft, -53.1 C
27,738 sdescentp.5
In the descent at 20,000 ft a 34 lb left handwheel input over 6 s moved the left aileron to 5.82 deg - the left circuit apparently snapped free of its binding; remainder of the flight flown from the left side with full aileron authority, roll disconnect still pulled; ambient above 0 C from 28,046 s

NTSB analysis

from the final report

The airplane departed in heavy rain and climbed to altitudes where the ambient temperatures were below freezing. During the flight, the crew recognized abnormal turn execution while on autopilot and disconnected the autopilot. Control wheel forces were excessive and aileron response was limited. The crew’s attempts at corrective action, including a roll disconnect to isolate the left and right aileron control circuits, were unsuccessful, and they elected to divert to an alternate airport. While descending, high control wheel force input by the flight crew resulted in the left aileron control returning to normal operation. A few minutes later, the ambient temperature rose to above freezing and an uneventful landing was made. Examination of the control system did not reveal any areas where binding had occurred; however, the events of the flight were consistent with water accumulating in the aileron control system, freezing at higher altitudes, and temporarily binding the aileron control system.