Date: 201501
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: PA-23-250 Aztec
Operator: Air Taxi
Operating under: Part 135
Flight plan: None
Mission: Passenger
Flight phase: Parked
ATC / advisory: Tower ZZZZ
Aircraft component: Gear Extend/Retract Mechanism
Component problem: Design; Malfunctioning
Reporter organization: Air Taxi
Reporter function: Captain; Pilot Flying; Single Pilot
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Confusion
Anomaly: Aircraft Equipment Problem Critical; Ground Event / Encounter Ground Strike - Aircraft
Detector: Person Flight Crew
When detected: Pre-flight
Result: Aircraft Aircraft Damaged; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Was to depart VFR on a PA23-250 AZTEC late afternoon. Boarded with two passengers and one crew (myself). Completed the safety briefings and then the pre-start checks. Upon cranking the right engine for start the left main gear collapse. When mechanics arrived and lifted the left wing up and placed it on a jack; the right gear then collapsed. After raising the right wing the nose gear started to retract; but was saved in time by tying the tail down. It is suspected that the anti-retraction valve or some valve inside the main hydraulic housing may be at fault.

Callback: Reporter stated all three landing gears showed down and locked with three 'Green' lights prior to his attempt to start the Right Engine. The twin engine PA23 AZTEC he was preparing to fly has only one Engine Driven Hydraulic Pump (EDP) compared to the 'F' Model AZTECs with two EDPs. Flaps and Landing gear are all hydraulically driven with the landing gears having hydraulic priority over the flaps. That decision is controlled by a priority valve inside a hydraulic module under the floor. The Anti-Retraction switch on the Left Main gear that he referenced in his report is similar to a Squat switch. Reporter stated their Engine Start procedures include placing the Flap handle in the 'Up' position before cranking the Right Engine with the only hydraulic EDP. That procedure would ensure the flaps were up and when hydraulic pressure reached approximately 600 psi; the flap handle would then pop back to the neutral position. With the Left Main Gear compressed; the Squat switch signal to the Hydraulic module should also keep the Anti-Retraction valve closed. After the Left Main collapsed and mechanics lifted the wing; they started pulling the Left gear aft; that's when the Right Gear collapsed. Both Main gears retract forward; not inboard or outboard. The Nose Gear retracts aft. If the single Engine Driven hydraulic pump fails; an Emergency Hand pump can be used to pump up hydraulic pressure. If that fails; a CO2 bottle will blow the gears down. Plans were to replace the Hydraulic module and ferry the aircraft to their Maintenance Base. Since his incident; another pilot told him about a similar event that had just occurred on that pilot's PA23 aircraft.



Synopsis: Pilot reports that upon cranking the Right Engine for start on a Piper PA23 Aztec; the Left Main Gear collapsed; followed by the Right Main Gear collapsing when mechanics had lifted the left wing. After mechanics raised the right wing; the Nose Gear started to collapse.