The Evolution of MSAW

with NTSB’s investigation of the UPS crash in Birmingham turning to the question of why ATC did not alert the pilots to their low altitude, the history behind MSAW is well worth reviewing. It all started in 1972…

The Accident:

On December 29, 1972, at 11:42PM, Eastern Airlines Flight 401 (EAL 401) crashed in the Everglades, west of Miami. There were 101 fatalities. The aircraft was a shiny new L-1011, having flown only four months. The flight from JFK to Miami was just minutes from their intended landing, and the crew lowered the landing gear, but one of the indicator lights did not illuminate. Assuming the worst (that the gear was not down and locked), the crew elected to abort the approach and resolve the indicator light problem. They coordinated with ATC and were issued a climb to 2,000′ and a flightpath out to a holding pattern over the Everglades. Somehow, in the next eight minutes, all three crew members (the captain, the first officer, and the flight engineer) became hyper-focused on solving the lightbulb problem, and nobody noticed that the autopilot had been inadvertently cut off.

Well, they noticed, but only seven seconds before impact. They were flying in a clear sky on a moonless night, over the dark Everglades. The lightbulb problem had been resolved, and they requested a return to the airport. At 2341:47, the FAA air traffic controller cleared them to return to Miami but said nothing about their deviation from the assigned altitude of 2,000′. They began their turn and fifteen seconds later , at 2342:05, the First Officer noticed and immediately commented about the altitude disparity. At 2342:10, the aircraft’s radar altimeter warning system began announcing with a series of ‘beeps’, which ended just two seconds later. They impacted terrain at 2342:12.

The NTSB Recommendation:

NTSB was established in 1967 and, at five years old, they were just getting into their groove, figuring out how to press FAA to fix safety problems. The EAL401 accident was thoroughly investigated and NTSB produced their final report, approved on June 14, 1973. They also produced a series of aviation safety recommendations, including:

  • Safety recommendations A-73-011 through A-73-013 were sent to the FAA Administrator on May 2, 1973. All three were marked ‘closed’ on July 3, 1974.
  • Safety recommendations A-73-039 through A-73-043 were sent to the FAA Administrator on June 12, 1973. A-73-039 was marked ‘closed’ on May 29, 1974. A-73-041 was marked ‘closed’ on August 8, 1979. A-73-040 was marked ‘closed’ on August 12, 1980.
  • Safety recommendations A-73-046 was sent to the FAA Administrator on June 14, 1973. A-73-046 was marked ‘closed’ on September 16, 1977.

The last of these safety recommendations, A-73-046, called for a review of the radar system “…FOR THE POSSIBLE DEVELOPMENT OF PROCEDURES TO AID FLIGHTCREWS WHEN MARKED DEVIATIONS IN ALTITUDE ARE NOTICED BY AN AIR TRAFFIC CONTROLLER.”

The Implementation of MSAW:

FAA’s actions are summarized within the PDF history document, ‘FAA Chronology, 1926-1996’. An entry shows that, in July 1974, they announced a contract with Sperry Rand UNIVAC to develop MSAW, and the first MSAW system was commissioned at LAX in November 1976. Here is a copy of the chronology item:

Nov 5, 1976: FAA commissioned the first Minimum Safe Altitude Warning (MSAW) system, an add-on computer software feature specially devised for use with the ARTS III radar terminal system, at Los Angeles International Airport. MSAW had the capacity to spot unsafe conditions by automatically monitoring aircraft altitudes and comparing them to terrain maps stored in the computer’s memory. If aircraft descended dangerously close to the ground, aural and visual alarms on their consoles alerted controllers who could then radio warnings to pilots (see Oct 28, 1977). Sperry Rand’s UNIVAC division developed MSAW under a contract announced by FAA on Jul 17, 1974. The need for such a system had been highlighted by the crash of an L-1011 near Miami (see Dec 29, 1972).