
NTSB: Dali had two blackouts on day before bridge strike
Written by Nick Blenkey
Photo: NSTB
Even as efforts continue to remove the containership Dali from beneath the remnants of Baltimore’s Francis Scott Key Bridge, an NTSB preliminary report has been released that says that the Dali lost power twice before slamming into the bridge.
Two other things that emerge is that all drug and alcohol tests were negative and that fuel tests revealed no fuel contamination.
What happened with the power blackouts — and in particular the two experienced while the ship was in port? If you’re unfamiliar with ship’s electrical circuitry, the New York Times coverage won’t insult your intelligence, but if you want the full technical nitty gritty,on the blackouts read on.
The figure below, taken from the report, shows a simplified one-line electrical diagram of the Dali electrical power distribution system. The breakers shown reflect their positions at departure on March 26.

The NTSB preliminary report says that ship’s electrical power was supplied by four alternating current generators, which were each driven by a diesel engine. Generator Nos. 1 and 4 were rated for 4,400 kW, and generator nos. 2 and 3 were rated for 4,000 kW. The generators were connected to a 6,600-volt high-voltage (HV) main electrical bus by the vessel’s power management system that powered various shipboard equipment, including the main engine lubricating oil pumps, the bow thruster and reefer containers.
A 440-volt low-voltage (LV) electrical bus was connected to the HV bus via redundant step-down transformers (TR1 and TR2 in figure 5). The LV bus powered vessel lighting and other equipment, including steering gear pumps and the main engine cooling water pumps. Breakers were located on either side of the step-down transformers—HR1 and HR2 on the HV side, LR1 and LR2 on the LV side. The LV bus could also be split with an installed bus tie (LVR in figure). The bus was designed to be normally operated with the LV bus tie closed, which was the configuration during the accident voyage.
BLACKOUTS IN PORT
The strike on the bridge happened after the vessel experienced two power failures in a sequence of events described in detail in the NTSB preliminary report. Those failures came after, on the previous day, about 10 hours before leaving Baltimore, the Dali experienced a blackout (loss of electrical power to the HV and LV buses) during in-port maintenance. In response, the crew made changes to the electrical configuration.
On March 25, says the preliminary report, while working on the diesel engine exhaust scrubber system for the diesel engine driving the only online generator (generator No. 2), a crewmember mistakenly closed an inline engine exhaust damper. Closure of this damper effectively blocked the engine’s cylinder exhaust gases from traveling up its stack and out of the vessel, causing the engine to stall. When the system detected a loss of power, generator no. 3 automatically started and connected to the HV bus.
Vessel power was restored when crewmembers manually closed HR2 and LR2. Generator No. 3 continued to run for a short period, but insufficient fuel pressure caused its speed to decrease, and its breaker (DGR3) opened; a second blackout (another loss of electrical power to the HV and LV buses) occurred. In the meantime, the crew had reopened generator No. 2’s engine exhaust damper, and the generator automatically restarted and then connected to the HV bus when DGR2 closed.
While recovering from this second blackout, the crew switched the bus configuration to use breakers HR1 and LR1 and the bus’s associated transformer (TR1) instead of breakers HR2 and LR2, which had been in use for several months. TR1 and its associated breakers, HR1 and LR1, were in use when the ship departed on March 26.
The first in-port blackout was caused by the mechanical blocking of the online generator’s exhaust gas stack. The second blackout in port was related to insufficient fuel pressure for the online generator. During both of these electrical power-loss events, the online generators’ breakers (DGR2 and DGR3) to the HV bus opened before the HR2 or LR2 breakers opened. During the recovery, the crew put TR1 online to feed the LV bus because TR2 had reportedly been in use for several months.
The first vessel blackout after departure on March 26 occurred when the HR1 and LR1 breakers opened unexpectedly.
The NTSB is still investigating the electrical configuration following the first in-port blackout and potential impacts on the events during the accident voyage.
The NTSB stresses that its preliminary report is just that, preliminary, and is subject to change.
More details in the full preliminary report: