The importance of proper training, procedures and pre-flight planning when flying in mountainous regions cannot be over emphasized.
In the Pacific area, the combined effect of the great mountain system and the adjacent Pacific Ocean lead to extremely changeable weather conditions and a variety of weather patterns. Some of the factors to be taken into consideration regarding the effect on aircraft performance when operating under these conditions include the following:
In the western mountainous region VFR routes may be marked by diamonds on visual navigation charts. The routes are marked for convenience to assist pilots with pre-flight planning. The diamond marks do not imply any special level of facilities and services along the route. Pilots are cautioned that the use of the marked routes does not absolve them from proper pre-flight planning or the exercising of good airmanship practices during the proposed flight. Alternative unmarked routes are always available, the choice of a suitable route for the intended flight and conditions remains the sole responsibility of the pilot-in-command.
Higher density altitude means thinner air. For pilots, the bottom line is that high density altitude means seriously degraded aircraft performance. Technically speaking, density altitude is true altitude adjusted for non-standard temperature and pressure. At high density altitudes, it is common for non-turbocharged airplanes to climb at only 200 or 300 feet per minute. Would you be comfortable taking off toward rising terrain with that kind of performance? There are many ways to determine density altitude. It is often included in a METAR. You can also calculate density altitude with your flight computer, or even with simple math. Density altitude increases approximately 120 feet for every one degree Celsius the temperature is above standard.
When the wind speed is above about 25 knots and flowing perpendicular to the ridge lines, the air flow can form waves, much like water flowing over rocks in a stream bed. The waves form down wind from the ridge line and will be composed of very strong up and down drafts, plus dangerous rotor action under the crests of the waves. If enough moisture is present, lenticular clouds can form to give a visual indication of the wave action. These clouds are reported in the remarks section of hourly sequence reports as ACSL (altocumulus standing lenticular) or CCSL (cirrocumulus standing lenticular).
Since flying direct to your destination is usually not feasible, preflight planning must include a careful evaluation of routes and altitudes. Around large mountains and ridgelines, it is usually best to plan a flight path that will follow lower terrain around the upwind side of the mountain. Going over the mountain may be feasible if your aircraft has the capability to fly high enough. When the winds aloft are 20 knots or greater, it’s best to fly at least 1,000 feet higher than the mountain peak you are crossing. Be prepared for updrafts on the upslope side and downdrafts on the downslope side. It is also a good practice to cross mountain ridges at a 45° angle. Doing so allows you to turn back with less than a 180° turn if unexpected turbulence or downdrafts are encountered.