Some of the more memorable flights Ive made over the years involve flying over or through mountainous terrain. The terrain itself, of course, is visually interesting, with vivid colors and shapes, contrasting with the overwhelming monotony other geographic areas may present. I will long remember a late afternoon, eastbound flight over New Mexico, with some of my favorite music blasting over the headphones as I watched the terrain underneath change to its nocturnal state. I was overflying the terrain, though: The airplane performed as it always does at 13,000 feet msl. The weather was utterly benign, yet Im glad I was cruising instead of taking off or landing. On another occasion, I found myself looking at decent weather but strong winds aloft for a flight over other portions of New Mexico, plus Arizona and Nevada. I scrubbed that flight due to the forecast winds at altitude, which were at or above the airplanes stall speed. But it wasnt the winds velocity that concerned me. Instead, it was the turbulence guaranteed to result from winds of those speeds flowing over and around the various terrain features over which I needed to fly: Ill take a smooth ride over a bumpy one, anytime. In addition to the discomfort of turbulence, theres a much higher workload involved. And, of course, theres the wear and tear on the airplane, along with the slightly higher risk of structural failure. That the airplane in that instance didnt have much excess power available at altitude also figured prominently in my decision to stay on the ground that day. Despite being able to count on the fingers of one hand the few occasions Ive had to make a high-density-altitude takeoff, I have a healthy respect for the operation. One could also say I know my and the airplanes limitations, and prefer avoidance instead of confrontation. That the number is so low stems from having done most of my flying in the eastern U.S., from good planning-leave early in the morning, before the air heats up-and from being able to overfly much of the high terrain and land at a lower-altitude destination. Call it “respect.” Of course, just because we have a healthy respect for an operation doesnt mean we can get away with engaging in it anyway. Background On August 5, 2007, at 0945 Pacific time, a Piper Cherokee 140 collided with terrain while attempting to return to the South Lake Tahoe (Calif.) Airport after taking off from Runway 18 moments before. The private pilot was killed, his passenger was seriously injured and the airplane was substantially damaged. Visual conditions prevailed. At 09:41:50, the pilot announced his takeoff and intention to make a left downwind departure. At 09:44:56, he stated was turning left crosswind. A witness reported seeing the airplane in a right turn about 20 feet above the tree tops, with a slightly nose-low attitude. The airplane then rolled to a level attitude heading in a northerly direction, and started to sink with the wings rolling left and right just prior to its collision with trees. Witnesses also described very windy and gusty conditions in the vicinity. Investigation The airplane wreckage came to rest at an elevation of 6316 feet msl in level terrain populated with mature pine trees. All major airframe components were located together; control continuity was established. The main wreckage was oriented on a bearing of 300 degrees magnetic laying on its right side. The left fuel tank was approximately half full of fuel. The fuel selector was set to the left tank position. The South Lake Tahoe airport is located in a valley basin. The Airport/Facility Directory establishes the airport elevation at 6264 feet msl; the airport has a single runway, oriented north-south. Mountainous terrain rises up on the western, southern and eastern sides, with a mountain peak of 9840 feet less than five miles east. An airport information sheet published by the airport manager included procedures for noise abatement, arrival and departure, and a caution for departing Runway 18. Copies were available at the pilot briefing area in the airport terminal. The departing instructions for Runway 18 state, “NO LEFT TURN OUT under 7500 msl…. Track the river to the golf course. Circle and climb to 7500 msl over the golf course. If not able to climb to 7500 feet consider returning to the airport.” An advisory for Runway 18 departures states, “CAUTION: Downdrafts are often encountered near the runway abeam the terminal building and west of the golf course near the mountains.” The local automated weather system observed southerly winds at 16 knots gusting to 31, a temperature of 69 deg. F and an altimeter setting of 30.10 in. Hg. The airport manager later stated the reported winds are usually less than what is experienced at other locations in the airport vicinity. Airplane gross weight was estimated to be 1965 pounds, within allowable limits. Density altitude was calculated as 8445 feet. Under those conditions, the best rate of climb was found to be 220 fpm. Probable Cause The National Transportation Safety Board determined the probable cause of this accident to include “The pilots inadequate preflight planning/decision, and his failure to maintain clearance from trees during takeoff-initial climb. Contributing to the accident were turbulence, downdrafts, and rising terrain.” The NTSB also could have explored the density altitude calculations and drastic performance reduction, plus slim likelihood of being able to obtain book performance from the airplane, especially on a gusty day. The Board did, however, quote from FAA publications addressing weather and mountain flying, noting aircraft engaged “in low-level flight operations over mountainous terrain in the presence of strong winds (20 kt or greater at ridge level) can expect to encounter moderate or greater turbulence, strong up- and downdrafts, and very strong rotor and shear zones. This is particularly true for general aviation aircraft.” We often have warned against flying relatively underpowered aircraft in mountainous regions, and taken some flak for it. In this instance, the airplane certainly didnt have an excess of power, but the pilot also was stacking the deck against him by choosing to fly in gusty conditions. That the airplane cant do what is asked of it often is hard to swallow for some pilots. But there are conditions in which even Boeings and Airbuses shouldnt be flying.
