NTSB: Dali Lost Power Because Breakers Tripped

Auxiliary engines kept running, but were no longer connected to the ship's systems

Dali entangled with Key Bridge
Image courtesy FBI Baltimore Field Office

Published May 14, 2024 8:09 PM by The Maritime Executive


The NTSB has released its initial factfinding report on the allision of the boxship Dali with the Francis Scott Key Bridge. While the agency has not drawn any conclusions yet about causal factors, the summary contains previously unreleased information about the two brief losses of electrical power and the loss of propulsion that preceded the accident. 

First, NTSB confirmed early reports that Dali had lost power twice at the pier on the day before she sailed. On March 25, Dali experienced a blackout during in-port maintenance due to human error. A crewmember accidentally closed an engine exhaust damper for the No. 2 generator, causing it to stall. The No. 2 was the only generator running (out of the four aboard), so the ship experienced a brief blackout until the crew could bring power back online with the No. 3 generator. After a short period, the No. 3 generator experienced loss of fuel pressure and its breaker opened, prompting a second blackout alongside the pier. 

During this incident, the crew switched over from using the No. 2 transformer to the No. 1 transformer (and, importantly, the No. 1 transformer's breakers). Either transformer could be used to turn the 6,000-volt power supply from the generators into "low voltage" 440-volt power for the ship's systems, and the No. 2 had been in use for months. 

Simplified diagram of Dali's high-voltage and low-voltage electrical busses (NTSB)

When the ship departed the pier early the next morning, the No. 3 and No. 4 generators were running, and the No. 1 transformer, No. 1 high voltage breaker and No. 1 low voltage breaker were in use. Most of the equipment that had caused problems at the pier the day before (save for the No. 3 generator) was offline.

The ship left the pier at 0036, released her docking tugs, made her turn and headed outbound in Fort McHenry Channel at a slow bell. An apprentice pilot had the conn with an experienced pilot watching. At about 0125, as Dali was about three ship lengths away from the Francis Scott Key Bridge, the No. 1 high and low voltage breakers both opened and cut off all power to the rest of the ship. The reasons are unknown and are under investigation, with help from the manufacturer. 

The two auxiliary engines kept running, uninterrupted, but they were no longer connected to the rest of the ship's systems. Without power to the electrically-driven lube oil pump and coolant pump for the main engine, the propulsion system automatically shut down. It was never brought back online. 

Rudder control was also temporarily lost, and the rudder was stuck amidships as the Dali drifted towards the bridge. The emergency generator started up shortly after and restored power to bridge systems and to one steering pump for rudder control. At 0126:13, the senior pilot ordered 20 degrees port rudder. This had a reduced effect on the ship's trajectory, since the propeller was no longer pushing water past the rudder.

The crew manually reconnected the high voltage and low voltage breakers, restoring full power to the vessel. At 0126:39, the pilots called for an urgent tug assist and ordered an anchor dropped. 

Shortly after, the ship lost electrical power again. This time, the diesel generator breakers for the No. 3 and 4 generators had opened, cutting off the still-running auxiliaries from the high voltage bus. The emergency generator stayed on and kept providing backup power to the bridge. The No. 2 generator was running on standby and quickly connected automatically, and the crew closed the breakers for the No. 2 transformer (the one previously in use the day before) to bring back low-voltage power for the second time. From the start of the second blackout to the restoration of power took about 30 seconds. 

Overview of the accident timeline (NTSB)

At 01:29:10, Dali struck the southwest pier of the Key Bridge's main span at 6.5 knots, dropping the entire six-span truss into the water. One crewmember sustained a minor injury when the bridge deck hit Dali's bow, six road crew workers were killed and one worker was injured. Salvage operations to refloat the Dali are ongoing, and litigation over responsibility for the disaster is just beginning.