What happened
The aircraft was operating on the Des Moines (DRG) arrival route as part of Buffalo Airways Flight 721. Air traffic control vectored the flight for an instrument landing system (ILS) approach to runway 1. Aside from necessary deviations to avoid thunderstorms, the descent into the terminal area proceeded without incident.
At 21:51:01, the pilot was advised that the aircraft was five miles from the outer marker (OM). The flight was given a final vector to intercept the ILS localizer at an altitude of 2,400 feet and was cleared for the approach. At 21:53:07, the crew reported passing over the OM inbound.
At that moment, the local controller warned the flight to check their altitude immediately, noting they were supposed to be at 2,400 feet. Shortly thereafter, a Minimum Safe Altitude Warning (MSAW) alert was activated. The controller informed Flight 721 of the alert and instructed them to climb to 2,400 feet. The crew did not respond by radio to either safety advisory.
The aircraft crashed approximately three miles short of runway 1. The designated altitude for the outer marker was 2,400 feet, while the decision height (DH) for the approach was 1,211 feet. Flight data recorder and radar data confirmed the aircraft passed the OM at an altitude of only 1,700 feet.
The investigation
Cockpit voice recorder analysis revealed that the first officer provided altitude information in 100-foot increments down to the decision height. However, the first officer did not advise the captain of the altitude error when passing the outer marker. Furthermore, the first officer did not report that the runway was not in sight at the decision height.
The ground proximity warning system (GPWS) did not provide any warning to the crew prior to impact.