Boeing 720B accident at Front Royal, Virginia, 26 Apr 1972

Accident narrative

On April 26, 1972, at approximately 1635 eastern standard time, a Northwest Airlines Boeing 720B and a Lockheed Convair 240 experienced a near midair collision at approximately 8,500 feet, 8 miles west of Front Royal, Virginia. The Boeing crew took evasive action to avoid the Convair. Two cabin attendants on the Boeing received medical attention for minor injuries. Both aircraft continued to their destinations without damage.

### The flights

Northwest Airlines Flight 78 was a regularly scheduled passenger flight from Seattle, Washington, to Dulles International Airport, operating under Instrument Flight Rules (IFR). There were 77 passengers, a flight crew of three, and four cabin attendants aboard. The captain had 15,618 hours of flying time, with 5,736 hours in the Boeing 720. The first officer had 2,908 hours, including 1,200 in the Boeing 720.

The Convair 240, operated by Lockheed Aircraft Corporation, was on a Visual Flight Rules (VFR) flight plan from Bradley International Airport in Windsor Locks, Connecticut, to Dobbins Air Force Base, Georgia. It carried five passengers and a flight crew of two. The pilot had 10,140 hours of flying time, and the copilot had 2,200 hours of civilian flying time and 3,300 hours of military flying time.

### The sequence of events

Flight 78 established initial contact with the Washington Air Route Traffic Control Center (ARTCC) at 1626:30 while descending. At 1631:05, the flight was cleared to descend and cross Front Royal at and maintain 8,000 feet.

The Convair 240 had climbed to 8,500 feet earlier in its flight and maintained that altitude. The aircraft was operating a transponder on the VFR code and was not in radio contact with Washington ARTCC, which was not required.

At approximately 1635, the first officer of Flight 78 saw a Convair-type aircraft at the 10 o'clock position, which appeared to be at the same altitude and on a converging course. He stated that he seized the controls and executed a descending left turn. The captain observed the first officer applying the control inputs and almost simultaneously glimpsed a two-engine transport passing overhead from about the 10 o'clock to the 4 o'clock position.

Aboard the Convair, which was on a heading of approximately 230 degrees magnetic, the copilot was startled to see a large aircraft passing below and to the right at the 3 o'clock position on an easterly heading. He estimated the vertical clearance at 300 feet. He placed his hands on the control column but did not disengage the autopilot or alter the flightpath. The Convair pilot stated he did not see the other airplane. Both pilots stated they immediately noted their altitude was exactly 8,500 feet. The Convair passengers were unaware of the near collision.

After returning to level flight, Flight 78 asked Washington Center if there was conflicting traffic; the Center advised negative. Flight 78 then reported the near miss to Dulles Approach Control, stating they had taken extremely evasive action. The flight landed at Dulles without further incident.

### What the investigation found

The Flight Data Recorder on the Boeing 720B recorded the evasive maneuver. The altitude trace showed a gain of 75 feet to 8,675 feet, followed by a descent to 6,075 feet. The heading turned left from 107 degrees magnetic to 90 degrees, then back to the right past the original heading. Static and flight recorder systems on the Boeing were checked and found to be within tolerances. A static system check on the Convair showed that with the test set corrected to 8,000 feet, the pilot's altimeter read 8,005 feet and the copilot's read 7,935 feet.

The pilot of an Air Force C-131 operating on an IFR flight plan at 7,000 feet stated he observed two airplanes pass approximately 4 miles ahead of his airplane near Front Royal. He estimated their altitude at 10,000 to 11,000 feet with 500 to 1,000 feet of vertical separation.

Weather at Dulles Airport at 1625 was reported as thin scattered clouds at 25,000 feet with 20 miles visibility. The Northwest crew reported scattered clouds at about 10,000 feet and in haze, while the Convair crew reported VFR conditions with visibility in excess of 5 miles in all directions.

The Washington ARTCC controller stated he was working five IFR flights near Front Royal at the time and did not see a VFR target in the area. The Board noted that while the controller's workload of five aircraft would not prohibit issuing a traffic advisory, the five beacon targets in a concentrated area may have prevented him from seeing the VFR target.

The investigation found that Washington Center was not complying with the Federal Aviation Administration's "keep-'em-high" program, which required terminal airspace to be configured so high-performance aircraft enter at 10,000 feet and remain there as long as possible before descending. The Convair pilot stated he had planned his VFR flight well west of Washington specifically to avoid jet traffic, believing the policy would keep arriving jets at higher altitudes. The Board noted that mountainous terrain west of Washington forces VFR traffic above 5,000 feet, and allowing IFR traffic to descend below 10,000 feet creates a collision hazard between controlled and uncontrolled aircraft.

Assuming a maximum in-flight visibility of 20 miles, the Board calculated that each aircraft could have been visible to the other for approximately 3 minutes and 20 seconds prior to the near collision. The Board stated this time frame and the relative positions should have provided ample opportunity for each crew to observe the other aircraft and take corrective action.

### Probable cause

The National Transportation Safety Board determines that the probable cause of this incident was the lack of visual scanning vigilance on the part of both flightcrews to provide safe in-flight separation while operating in VFR flight conditions.