N73670Cessna 172N · 1976

On May 11, 2022, about 1235 mountain daylight time, a Cessna 172N airplane, N73670, was destroyed when it was involved in an accident near Broomfield, Colorado. The pilot sustained fatal injuries. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 personal flight.

The pilot was working on obtaining his commercial pilot certificate via a 14 CFR Part 141 approved course and was performing a local area flight when the accident occurred. A review of automatic dependent surveillance - broadcast data indicated that the airplane departed from the Rocky Mountain Metro Airport (BJC), Broomfield, Colorado, to the southeast and entered the traffic pattern at the Colorado Air and Space Airport (CFO), Watkins, Colorado. The pilot performed a landing at CFO and then departed to the southeast. The pilot performed flight maneuvers to the south of Bennett, Colorado, and then proceeded west to BJC. The pilot performed a touch-and-go landing to runway 12R at BJC and remained in the traffic pattern.

The air traffic controller instructed the pilot to widen his downwind leg before turning base for runway 12R due to traffic landing on runway 12L. The controller then changed the landing to runway 12L and cleared the pilot to land. The pilot performed a right turn to the base leg, and after the airplane was established on final for runway 12L, it turned abruptly to the north and descended rapidly.

The airplane impacted an intersection about ½ nm northwest of the approach end of runway 12L. The airplane impacted the ground and a traffic light pole and then came to rest on a sidewalk. A postimpact fire ensued and destroyed the airplane.

Two witnesses, one who is a designated airworthiness representative (DAR) and the other who is a Federal Aviation Administration (FAA) aircraft certification specialist with an aerospace engineer and pilot background, observed the accident sequence while at a nearby golf course. They reported the airplane was not in an aerodynamic stall or spin. The DAR reported that the airplane appeared to be in a “gentle” left-hand turn and descent, with about 15-20° of bank. They did not observe any flight control movements, nor was there any smoke or fire coming from the airplane. They further reported that there were no abnormal engine noises and that the airplane was structurally intact while in flight.

A review of local area security camera footage showed the airplane was established on final approach for runway 12L. The airplane then turned abruptly to the north before it descended rapidly and impacted the ground.

The pilot was enrolled at the Western Air Flight Academy, which is based at BJC. He started his training for a commercial pilot certificate on January 11, 2022.

A review of the pilot’s logbook found that the pilot had received a pilot-in-command endorsement for a high-performance airplane on January 5, 2022, and he had received 5 hours of ground instruction for mountain flying and high-altitude operations. Due to the fire damage sustained to the logbook, the date for the ground instruction for mountain flying and high-altitude operations could not be determined.

The pilot’s logbook also showed that he had flown in the accident airplane multiple times in the past.

The pilot worked as a professional truck driver. He passed a Department of Transportation commercial driver’s license medical examination in April 2020.

An examination of the airplane’s maintenance records revealed no evidence of any uncorrected mechanical discrepancies with the airframe or the engine.

A High-Resolution Rapid Refresh model sounding identified possible low-level wind shear close to the surface near the accident site.

The National Weather Service issued an Area Forecast Discussion at 1115 and stated in part:

Winds will be the greatest operational impact today with VFR conditions. Winds will increase out of the southwest with speeds up to 25 kt then increase after 18z up to 32 kts for DEN and APA with BJC topping out around 25 kts. Winds will shift more westerly after 14z and be due west after 16z with gusting up to 32 kts.

The estimated density altitude for BJC at the time of the accident was 8,578 ft above msl.

An examination of the airplane’s maintenance records revealed no evidence of any uncorrected mechanical discrepancies with the airframe or the engine.

The wreckage was recovered from the accident site and was transported to a salvage facility for further examination. Most of the airframe was consumed by the postimpact fire. No bird remnants were observed in the wreckage. Flight control continuity was established for all of the flight controls. The flaps were found positioned about 30° at the time of impact. There was no evidence of an asymmetric flap deployment.

The engine was separated from the airframe and intact. Cylinder No.1 sustained impact damage. The propeller showed chordwise scratching on one blade. The other blade was bent, about midspan, to the rear about 90°. During the examination there were no signs of a preimpact mechanical malfunction or failure with the airframe or the engine that would have precluded normal operation.

Low-Level Wind Shear

The FAA has published Wind Shear FAA-P-8740-40. This document discusses low-level wind shear and states in part:

Another type of surface obstruction—mountains—can also affect wind shear. Some airfields are close to mountain ranges, and mountain passes are close to the final approach paths. Strong surface winds blowing through these passes can cause serious localized wind shear during the approach. The real problem with such shear is that it is almost totally unpredictable in terms of magnitude or severity. A pilot can expect such shear whenever strong surface winds are present.

Density Altitude

The FAA has published Density Altitude FAA-P-8740-2. This document discusses density altitude and states in part:

Density altitude is formally defined as “pressure altitude corrected for nonstandard temperature variations.”

The formal definition of density altitude is certainly correct, but the important thing to understand is that density altitude is an indicator of aircraft performance. The term comes from the fact that the density of the air decreases with altitude. A “high” density altitude means that air density is reduced, which has an adverse impact on aircraft performance.

Whether due to high altitude, high temperature, or both, reduced air density (reported in terms of density altitude) adversely affects aerodynamic performance and decreases the engine’s horsepower output.