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When the air meets a mountain on the windward side, it’s fairly smooth. However, as it “climbs” the mountain, lifting develops and the wind picks up speed, reaching its maximum at the summit. As it passes the crest, the flow becomes more complicated, spawning lots of devious downdrafts.
The San Jacinto Mountains, west of Palm Springs, CA are famous for their Leeward Pull. In fact, the downdrafts can be a powerful menace to pilots. In 1979, I was flying a Beech Bonanza A-36 and two clients from Phoenix, AZ to Van Nuys, CA. ·I was on an IFR flight plan because of the rain in the LA basin. As I flew west from Palm Springs on the victor airway, the airspeed decreased.
Many times before, I had experienced a decrease in airspeed on the leeward side of those mountains, but I had only lost about 20 to 30 knots. Usually, as I continued past the mountain’s crest, the speed would be safely restored. This time, it was not a 20 to 30 knot decrease. The autopilot maintained the altitude, but the speed was decreasing towards a stall. I waited for the mountain to return my speed, but it wasn’t in a giving mood.
I heard the stall warning horn’s sickening buzz, and I told the air traffic controller that I could not maintain the MEA. I had no choice. I could stall, or start a descent and turn. The controller suggested that I try the victor airway just a few miles to the south. This airway started at Thermal, CA. It also required a lower MEA. The controller and I thought we could safely cross the San Jacinto Mountains on that route.
As we approached the ridge line on the new airway, the airspeed once again decayed and approached stall speed. The Leeward Pull was determined to claim us to the mountains.
“Manana, amigo”, the mountains seemed to say. “Come back Manana. My wind is just too strong for your A-36.”·
Manana Van Nuys! I did a 180 before the mountains and the leeward pull could claim our lives, and headed for Palm Springs. After refueling, I returned to Phoenix with two disappointed clients. They were, however, very grateful that we were still alive, and not one with the San Jacintos.
Sometimes, pilots aren’t as lucky as I was in 1979. On December 14, 2000, at 2116 MST, a Mooney M20K, N5772C, was destroyed when it impacted Mt. Ogden (Snow Basin Ski Area) near Ogden, Utah. The instrument rated private pilot was fatally injured. Night instrument meteorological conditions (IMC) prevailed at the accident site at the time of the accident. The National Weather Service recorded intermittent with scattered to broken conditions cloud cover from 8,000 feet MSL to 14,500 feet MSL. AIRMET Sierra was issued at 1700 MST for the Rocky Mountain Area, (including all of Utah), warning pilots of IFR conditions and mountain obscuration. It was a whiteout at Snow Basin, with near zero visibility and the strong winds from the west, southwest, gusting at 34 knots, created horizontal snow.
The pilot started his VFR cross-country flight from Kansas City International Airport 5 hours 27 minutes before the accident. The pilot did not file a flight plan, but availed himself of flight following services.
At 2107 MST, the pilot started a 500 minute rate descent from 14,500 feet into Ogden. At 2114:46, the pilot told Salt Lake ARTCC, “I’ve got Ogden in sight,” and he cancelled VFR flight following. The last radar return at 9,400 feet MSL was recorded at 2115. Calculations based on the airplane’s ground speed indicate that it impacted the mountain at 2116 MST. ·
The strong leeward pull had sucked him mercilessly into the east side of Ogden Mountain, (elevation 8,885 feet), striking just 40 feet below its crest. The wreckage slid another 760 feet down the mountain.
What can we learn from these two events?
If you absolutely need to be somewhere, then general aviation isn’t the way to travel. Even with an IFR ticket and approved ice protection on your aircraft, sometimes you just can’t get there.
- IFR and mountain obscuration AIRMETs are to given a great deal of respect
- If you encounter inadvertent IFR, do a 180 and get out of there!
- 20+ knots blowing over the mountains can be one of Mother Nature’s more treacherous traps.
MOUNTAIN WAVE TRAVEL TIPS
- Density altitude increases when the temperature INCREASES and/or the atimeter DECREASES.
- High density altitude decrease WING, PROP, AND ENGINE performance.
- When flying in a canyon, fly on the windward, or upwind side of ridges. Downdrafts are less likely to be found there.
- Approach a ridge line on a 45o angle and depart it on a 90o angle. If you encounter a dangerous downdraft, and you’re on a 45, instead of turning 180 degrees, you’ll only need to turn 45 degrees to become parallel to the ridge, and on your way to safety.·
- Always cross a ridge line with at least a 1,000 foot margin it. If the winds are stronger than 20 knots, consider 3 to 4,000 above the ridge line for the crossing.
- Mountain winds greater than 20 knots: Fasten your seat belt.
- Mountain winds greater than 30 knots: Consider flying another day.
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