Incident Overview
On February 9, 2015, about 2207 central standard time, US Airways Flight 1825, an Embraer ERJ-190-100-IGW (registration N953UW), landed with the nose landing gear not extended at George Bush Intercontinental Airport (IAH), Houston, Texas. The airplane, operating under 14 CFR Part 121 as a scheduled domestic passenger flight, had departed Philadelphia International Airport (PHL) about 1706 en route to IAH. Night visual meteorological conditions prevailed, and the flight operated under an instrument flight rules flight plan.
The flight crew consisted of 4 crew members and 51 passengers. None were injured during the landing; however, one passenger received minor injuries during the subsequent evacuation.
Sequence of Events
According to the operator, when the flight crew placed the landing gear lever in the down position near IAH, they received amber caution and red warning messages on the engine indication and crew alerting system. After performing quick reference handbook items, the crew could not determine the landing gear position and conducted two passes by the air traffic control tower. Tower personnel reported that the nose landing gear (NLG) was not extended. The crew declared an emergency, and the captain landed the airplane on runway 27 with the NLG retracted. The forward fuselage contacted the runway, and the flight deck filled with smoke.
Post-Accident Findings
Investigation revealed that the NLG was rotated within the nose gear bay, causing it to jam and preventing normal extension. Several NLG and nosewheel steering system components were removed for examination.
The failure to extend was linked to an inadvertent nosewheel steering command from the nose wheel steering control module (NWSCM) following an undetermined failure. The NWSCM contains a drift monitor function that keeps the nose gear centered from liftoff to stowage. This function operates continuously when the NLG is weight-off-wheels but cannot command steering when the NLG is fully retracted due to lack of hydraulic pressure. When landing gear extension was commanded, steering hydraulic pressure restored, and the NLG rotated due to the inadvertent command.
Contributing factors included misrigged nose gear steering feedback sensors (resolvers), which caused the NWSCM drift monitor to become active, as indicated by digital voice and data recorder (DVDR) data showing resolver deflection after liftoff. The drift monitor remained active during flight with the gear retracted.
Additionally, the NWSCM experienced an undetermined failure during cruise, causing its control channel to halt internal communication. The NWSCM did not fail safely; a steering command (electrical current) remained as a "last command." When the crew commanded gear extension, an uncommanded steering actuation occurred as hydraulic pressure reached the NLG steering actuator. This caused the NLG to rotate prematurely and jam in the bay. The transition of the weight-off-wheels discrete signal indicated the gear had turned.
Examination and Service Bulletins
The NWSCM showed "no fault found" when subjected to the manufacturer's acceptance test protocol. No nosewheel steering rigging data was found in the NWSCM's nonvolatile memory. However, US Airways maintenance documentation related to an engineering order for the NWSCM installation indicated successful rigging, which would have included recording rigging data into memory.
The investigation identified commonalities between this event and two previous events, as well as one subsequent event. In response, Embraer issued service bulletins:
- Service Bulletin 170-32-0076 (original 09/Mar/15, Rev 03 29/Apr/16) and Service Bulletin 190-32-0063 (same dates) for checking steering rigging and adjustment.
- Service Bulletin 170-32-0082 (original 08/Apr/16, Rev 01 24/Jun/16) and Service Bulletin 190-32-0070 (same dates) to replace existing NWSCM part numbers with Part Number 1855A0000-06, incorporating modified software.