N953UW

On February 9, 2015, about 2207 central standard time, an Embraer ERJ-190-100-IGW airplane, N953UW, landed with the nose landing gear not extended at the George Bush Intercontinental Airport (IAH), Houston, Texas. The 4 crew and 51 passengers were not injured; one passenger received minor injuries during the subsequent airplane evacuation. The airplane was registered to and operated by US Airways under the provisions of Title 14 Code of Federal Regulations Part 121, as a scheduled domestic passenger flight. Night visual meteorological conditions prevailed for the flight, which operated on an instrument flight rules flight plan.

US Airways flight 1825 departed Philadelphia International Airport (PHL), Philadelphia, Pennsylvania, about 1706, destined for George Bush Intercontinental Airport (IAH), Houston, Texas. According to the operator, the flight crew reported that when they 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 accomplishing quick reference handbook items, the crew could not determine the position of the landing gear and conducted two passes by the air traffic control tower; tower personnel indicated that the nose landing gear (NLG) was not down. 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.

After the accident it was discovered that the nose landing gear (NLG) was rotated within the nose gear bay, which jammed the NLG within the bay and did not allow the nose gear to extend normally. Several components related to the NLG and nosewheel steering system were removed for examination and testing.

The failure of the NLG to extend was related 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 to keep the nose gear centered from the time between nose gear liftoff and when the gear is stowed after retraction. The drift monitor operates continuously when the NLG is weight off wheels but cannot command a steering response when the NLG is fully retracted due to lack of steering hydraulic pressure when the NLG is stowed. Once the landing gear was commanded to extend, steering hydraulic pressure was restored and the nose landing gear rotated due to the inadvertent command from the NWSCM. The command was related to the following combination of factors.

The nose gear steering feedback sensors (resolvers) were misrigged to the point that the nosewheel steering control module's (NWSCM) drift monitor function became active, as the digital voice and data recorder (DVDR) data indicated deflection of the NLG resolvers after lifting off the runway. (The drift monitor function, which aligns the NLG before retraction into the wheel well, remains active when the NLG is fully retracted but does not turn the gear when stowed due to the lack of hydraulic pressure.) As designed, the drift monitor function remained active during flight with the landing gear retracted.

The NWSCM experienced an undetermined failure during cruise flight that resulted in the NWSCM control channel halting internal communication. However, the NWSCM did not fail in a "safe" mode; a steering command (electrical current) remained active as a "last command."

When the flight crew commanded the landing gear extension during the approach to landing, an uncommanded steering actuation occurred as hydraulic pressure was applied to the NLG steering actuator. The hydraulic pressure caused the NLG to prematurely rotate; it then became jammed inside the landing gear bay. The transition of the discrete weight off wheels (WOFFW) from TRUE to FALSE indicated the gear had turned.

The NWSCM showed “no fault found” when subjected to the manufacturer's acceptance test protocol.

No nosewheel steering rigging data was found in the contents of the NWSCM's nonvolatile memory. However, US Airways maintenance documentation related to an engineering order for the NWSCM installation on the airplane indicated that a successful rigging was accomplished. A successful nosewheel rigging would have included recording the rigging data recorded into the NWSCM memory.

The investigation determined general commonalities between this accident event, two previous events, and one event that occurred after this event.

During the course of the investigation, Embraer issued service bulletins with actions to prevent the circumstances that led to the failure scenario that occurred in this event.  Service Bulletins 1 and 2, below, were issued for a check of steering rigging condition and adjustment, if necessary:

Service Bulletin 170-32-0076: Original issue date 09/Mar/15; Current Revision 03 dated 29/Apr/2016.

Service Bulletin 190-32-0063: Original issue date 09/Mar/15; Current Revision 03 dated 29/Apr/2016.

Service Bulletins 3 and 4, below, were issued to replace the existing NWSCM part numbers with Part Number 1855A0000-06, which incorporated modified software:

Service Bulletin 170-32-0082: Original issue date 08/Apr/16; Current Revision 01 dated 24/Jun/2016.

Service Bulletin 190-32-0070: Original issue date 08/Apr/16; Current Revision 01 dated 24/Jun/2016.