Delta Air Lines Flight 723

Delta Air Lines Flight 723 was a flight operated by a McDonnell Douglas DC-9 twin-engine jetliner, operating as a scheduled domestic passenger flight from Burlington, Vermont, to Logan International Airport in Boston, Massachusetts, with an intermediate stop in Manchester, New Hampshire. On July 31, 1973, at 11:08a.m., while on an instrument landing system (ILS) instrument approach into Logan in low clouds and fog, the aircraft descended below the glidepath, struck a seawall, and crashed. 88 of the 89 occupants aboard were killed.

Aircraft and crew
The DC-9-31, registration serial number 47075, was manufactured in September 1967 and had 14,639 flight hours at the time of the accident. The jetliner was one of the aircraft that Delta Air Lines acquired in their 1972 merger with Northeast Airlines, to whom the aircraft was originally delivered. The flight crew consisted of Captain John Streil (49) and First Officer Sidney Burrill (31). Captain Streil, a highly experienced pilot, had accumulated roughly 14,800 flight hours throughout his flying career. He had 17 years of experience as pilot-in-command and had been flying DC-9s since 1970, with 1,457 hours logged in them. First Officer Burrill was also an experienced airman, with just under 7,000 flight hours, including more than 200 hours on the DC-9. Occupying the cockpit jumpseat was a third pilot, Joseph Burrell (52), who was in training and was not yet qualified on the DC-9.

Accident sequence
The aircraft, flying at 3000 ft, had been vectored by Boston's approach control to intercept the final approach course to the ILS runway 4R approach at a 45-degree angle, about 2 nmi outside the outer marker. As it was later revealed, the controller was busy handling a potential collision conflict between two other aircraft, and therefore neglected to clear Flight 723 for the approach. The flight crew had to ask the controller for approach clearance, which was immediately given, but by that time—more than a minute after the intercept vector had been issued—they were high and fast and almost over the outer marker. The flight crew subsequently failed to stabilize the aircraft's descent rate and airspeed, descended below the glideslope and drifted left of the localizer course, hitting a seawall about 165 ft to the right of the extended runway centerline, about 3000 ft short of the runway's displaced threshold. The aircraft was destroyed, killing 87 of its 89 occupants. One of the two survivors died after two hours, and the other a 20-year-old sergeant in the U.S. Air Force, died of burn injuries on December 11, 1973. Chouinard is not listed by the National Transportation Safety Board (NTSB) as an official Delta 723 fatality due to requirements in the Code of Federal Regulations which, in 1973, defined a crash fatality as taking place within seven days of an accident. The rule was subsequently changed in 2018, such that death must occur within 30 days of a crash. Even under the revised rule, Chouinard would not be listed, as he died more than four months after the accident.

The weather conditions at the time of the crash were partial obscuration and fog, with a ceiling of 400 ft, 1/2 mile visibility, and light winds. Runway visual range (RVR) was 1400 to 6000 ft.

Investigation
The NTSB investigated the accident and was able to retrieve both the cockpit voice recorder (CVR) and flight data recorder (FDR). The investigators concluded that, based on the retrieved flight data and simulations, the flight crew very likely operated the flight director improperly, inadvertently switching it into a "go around" mode during the final approach, instead of the appropriate approach mode. This caused confusion and additional pressure, and contributed to the unstablized approach and deviation from the glide path. According to the CVR, no altitude callouts were made by the crew during the final approach, as the aircraft descended below the glideslope and decision height, until it struck the seawall and crashed.

The board determined the following probable cause for the accident:"...the failure of the flightcrew to monitor altitude and to recognize passage of the aircraft through the approach decision height during an unstabilized precision approach conducted in rapidly changing meteorological conditions. The unstabilized nature of the approach was due initially to the aircraft's passing the outer marker above the glide slope at an excessive airspeed and thereafter compounded by the flightcrew's preoccupation with the questionable information presented by the flight director system. The poor positioning of the flight for the approach was in part the result of nonstandard air traffic control services."