Cessna 421A accident at Nogales, Arizona, 22 Jan 1977 (N999MB)

Accident narrative

On the morning of January 22, 1977, a Cessna 421A, registration N999MB, crashed in mountainous terrain about 21 nautical miles north of Nogales, Arizona. The aircraft was operating on an instrument flight rules (IFR) flight plan. Both occupants were killed, and the aircraft was destroyed.

### The flight

The noncommercial business flight was scheduled from Nogales International Airport to Fresno, California. The pilot and one passenger were on board. The pilot held a private pilot certificate with single-engine, multi-engine land, and instrument ratings, and had reported 5,000 total flight hours on his latest medical application.

Before departure, the pilot contacted the Tucson Flight Service Station (FSS) to file an IFR flight plan. He requested a route from Nogales direct to Flats Intersection, intercepting Victor airway 66 to Gila Bend, and then via airways to Fresno, with an initial level-off altitude of 10,000 feet. The pilot stated he preferred to get his clearance on the ground because, although it was visual flight rules (VFR) at Nogales, he would "probably get into it pretty shortly afterward."

The FSS specialist informed the pilot that he could not find Flats Intersection on his chart and suggested entering the route into the computer as Nogales direct to Tucson. The specialist told the pilot that once airborne, the controller could provide a radar vector to the requested airway. The pilot agreed to the change.

The Tucson FSS called the Davis-Monthan Radar Approach Control (RAPCON) for the clearance. The flight data controller noted the computer-derived route was Nogales direct to Tucson at 10,000 feet. He asked the facility's assistant chief for assistance. Based on the assistant chief's advice, a clearance was issued and relayed to the pilot: cleared as filed to maintain 10,000 feet, maintain VFR until reaching 9,000 feet, and contact Tucson approach after departure. The flight departed Nogales about 0832.

### Sequence of events

At 0837:15, the pilot established radio contact with the RAPCON departure controller, reporting that he was climbing through 9,000 feet and was still VFR. The aircraft was not yet in radar contact. The controller cleared the flight to maintain VFR and climb to 11,000 feet. The pilot replied that he would not be able to do that. The controller then offered 10,500 feet, stating it was the minimum altitude he could use over Nogales. The pilot acknowledged, and the controller again cleared the flight to maintain VFR.

At 0838:22, the pilot informed the controller he was unable to maintain VFR. The controller stated he could not approve IFR at that low an altitude over Nogales and instructed the pilot to maintain VFR. The pilot stated he would have to descend. The controller asked if he was heading north toward Tucson, and the pilot responded he was on a heading of 360 degrees magnetic, 11 nautical miles from Nogales.

At 0838:52, the controller established radar contact 31 nautical miles south of Tucson. He asked the pilot to confirm he was VFR and advised that there was a peak directly in front of him, five miles away. The pilot replied he was not VFR. The controller then cleared the flight to turn left immediately to a heading of 180 degrees and maintain VFR, advising again that there was a peak in excess of 9,000 feet directly ahead. The pilot responded, "Turning to one eight zero."

At 0840:10, the controller asked how the pilot heard him. The pilot's response at 0840:15 was partially unintelligible. The controller asked if he could maintain VFR, but received no response. Further attempts to contact the flight were unsuccessful, and radar contact was lost.

### What the investigation found

The aircraft wreckage was found in a box canyon at an elevation of 5,600 feet, about five statute miles south of Mt. Wrightson. The airplane had struck the terrain in a right wing-low, nose-up attitude on a 095-degree heading. The aircraft was destroyed by impact and a post-crash fire. The propeller blades of both engines showed heavy damage, including bends, large nicks, and chordwise scratches.

Weather forecasts for the area called for clouds in merging layers with mountains mostly obscured. Witnesses near Nogales reported low clouds and obscured mountain tops between 0715 and 0900.

The investigation found that Nogales International Airport had a published IFR departure procedure requiring a climb to Flats Intersection, which had a minimum altitude of 9,000 feet. The direct route to Tucson, which the FSS specialist had substituted, had no low-altitude airway and crossed Mt. Wrightson, the highest terrain between the two points at 9,453 feet.

FSS specialists and RAPCON controllers testified they had little experience with IFR departures from Nogales. The FSS specialist did not know how to insert Flats Intersection into the ATC computer, though it was later discovered this could be done using radials and distance measuring equipment. The RAPCON assistant chief stated he assigned the 10,000-foot altitude because it was requested and he assumed the pilot would fly the published departure procedure to the northwest, where 10,000 feet would have been safe.

The Board found that the controller's issuance of a VFR climb restriction did not comply with the ATC Handbook, as the pilot had not requested it. Furthermore, the 10,000-foot altitude assignment did not meet the 2,000-foot terrain clearance requirement for mountainous areas on the direct route to Tucson. The Board noted that when the pilot reported he could not maintain VFR, the controller should have cleared him to climb in IFR conditions rather than extending the VFR restriction, which delayed the controller's consideration of other actions.

Radar data showed the aircraft began a left turn when instructed. One minute later, the data block showed an altitude of 8,600 feet, further south of its original position. The crash site was 3,400 feet below the altitude at which the avoidance turn was started, and on a 095-degree heading. The Board concluded that the pilot either became spatially disoriented during the left turn or allowed the aircraft to enter a spiral, and that the aircraft crashed during an attempted recovery.

The Board also found that the pilot was responsible for familiarizing himself with the terrain along his new route and did not check the amended clearance for proper terrain clearance altitudes. Additionally, the Board noted that official guidelines in the Airman's Information Manual and ATC Handbook were unclear regarding whether a pilot must inform ATC of their intent to use a published IFR departure procedure.

### Probable cause

The National Transportation Safety Board determined that the probable cause of the accident was "the controllers' issuance of an improper departure clearance, climb restriction, and altitude clearance." The Board found that the controllers' lack of knowledge and noncompliance with standard ATC procedures placed the aircraft in proximity to high terrain, and the pilot lost control of the aircraft for unknown reasons while executing an emergency, controller-directed turn.

Contributing to the accident were the inadequacy of official guidelines concerning the use of published IFR departure procedures, the failure of the departure controller to provide appropriate services, the inability of the flight service specialist to insert the pilot's requested departure route into the ATC flight data computer, and the failure of the pilot to check the new departure clearance and route for proper terrain clearance altitudes.