B737-700
Flight Crew Last 90 Days 145; Flight Crew Type 500
Situational Awareness; Distraction
Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
General Flight Cancelled / Delayed; General Maintenance Action
Aircraft; Human Factors; Procedure
While in the cockpit and waiting for an amended flight Release; due to added MEL requiring unpressurized flight; ground crew notified the cockpit that smoke was coming from the wheel well and intensifying. I called Ground Control to tell them that heavy smoke was coming from the wheel well and to notify and send the Fire Department to Gate. I depowered the aircraft; confirmed that no one was aboard; and exited through the forward entry door.[Recommend] Turn off electric hydraulic pumps if longer than normal time is taken to generate paperwork. The reason for the delay in getting the new Release was that ATC would not approve low altitude routing required when having to fly unpressurized.  Smoke in wheel well caused by overheating electric hydraulic pump.
This flight was a maintenance reposition flight from ZZZ to ZZZ1. There were four people aboard: Captain; First Officer; two Maintenance Technicians returning to ZZZ1. We were minutes away from pushing back from the gate; APU running; brakes set; in contact with our ground crew; and awaiting final paperwork.The ground crew notified us there was black smoke coming out from underneath the aircraft. I jumped up and looked out the forward entry door back towards the wheel well and saw nothing. I then opened the forward service door and saw black smoke billowing out from the bottom of the fuselage. I yelled to a Company person on the ground to call the Fire department. I turned and told everyone to get off the airplane. One of the Maintenance Technicians recognized the smell of the smoke as hydraulic related and said to shut off the hydraulic pumps. The First Officer turned them off as well as shut down the APU and all power to the plane. All four of us exited the aircraft via the jet bridge. I ran to the top and told the Agent to call the Fire department.When the Fire department arrived; the smoke had greatly dissipated. They eventually cleared the maintenance crew back in to the suspect area. Our Technicians confirmed it was an electric hydraulic pump that had released fluid via a relief valve that had then come into contact with overheated parts; creating the smoke.I do not know how to prevent pumps from malfunctioning. I was surprised how much smoke resulted from the overheated pump and fluid. Electric hydraulic pump overheated and began smoking prior to pushback at gate.
Two pilots report they were notified by ground crew personnel that heavy black smoke was coming out from the Main wheel well of their B737NG aircraft. Maintenance Technicians recognized the smell of the smoke as hydraulic related and instructed pilots to shut off the electric hydraulic pumps.