Mountain Flying Part 2

Category: 
Aviation
Page Last Modified / Reviewed: 
Nov 2019

 

Continued from: Mountain Flying Part 1

Orographic Lifting

As the wind blows moist air upslope, it will cool and may form clouds. If, as is often the case in winter, the air is stable, the clouds will stay close to the mountain, forming a "cap" cloud. However, if the air is unstable, as is usually the case in summer, this initial lifting will be enough to start convection and result in thunderstorm formation.

Microbursts

Wet microbursts are typically found in the middle of an active thunderstorm or intense rain shower and avoiding the strong downdraft is relatively easy. Dry microbursts, however, are more insidious because they occur with little or no warning in the clear air beneath virga. Dry microbursts are common in and near the Rockies and other mountainous areas of the western U.S. in the summer. Dry microbursts are likely when thunderstorms with bases above about 3000’ to 5000’ AGL (above ground level) exist and the temperature/dew point spread on the surface is more than about 40 degrees Fahrenheit. A good indicator of a dry microburst is when you see dust blowing underneath the thunderstorm. Recommend staying clear until the event passes (usually a few minutes).

Density Altitude

Density altitude is pressure altitude corrected for temperature. Higher density altitude reduces overall performance of the airplane. At higher density altitudes, takeoff and landing distances are increased, thrust is decreased, rate of climb and actual service ceiling are decreased, true airspeed (TAS) is higher for a given indicated airspeed (IAS), and turning radius is larger for a given IAS (due to higher TAS). To help regain performance at high-density altitudes, consider reducing aircraft weight (retardant and/or fuel load). Check your aircraft flight manual (AFM) performance data charts for takeoff and landing distances, climb rates, etc. Since your TAS is higher for a given IAS, many pilots respond to the visual cues of higher ground speed on takeoff by rotating at a lower IAS than normal. Rotating at too slow an airspeed may cause the airplane to take an even longer ground run than necessary. Turning radius is proportional to the square of TAS. For example, if you increase your TAS by only 10%, your turn radius will increase by 20%. In the fire pattern, this may result in an overshooting turn to final with the resultant last-minute corrections, rushed approach, etc. If in doubt, go around. Higher density altitudes also affect best rate and angle of climb airspeeds. Refer to your AFM to be sure you are flying the correct airspeeds to get the performance you expect. Be extra cautious about getting slow at high-density altitudes. Throttle response will be delayed (due to less dense air) and thrust is reduced due to less air over the prop blades. Stalls at high-density altitudes and close to the ground can be devastating with insufficient time/performance response to recover.

Ridge and Pass Crossing

A good technique is to cross ridges or passes at the ridge elevation plus at least 1000’ AGL. If the winds at mountain top level are above 20 knots, increase to 2000’ AGL. Plan to be at that altitude at least three miles before reaching the ridge and stay at that altitude until at least three miles past it. This clearance zone will give you a reasonable safety zone to avoid the most severe turbulence and downdrafts in windy conditions and/or the ability to turn the aircraft around in a descending turn if necessary. If conditions or airplane performance dictate, you may need to fly along the windward side of a ridge to find updrafts for gaining altitude before crossing a ridge. You may also need to circle before reaching the ridge if climbing out of a valley airport. Cross ridges at a 45° angle. This allows you to turn away from the ridge quicker if you encounter a severe downdraft or turbulence. Once you have crossed the ridge, turn away from it at a 90° angle to get away from the most likely area of turbulence quickly. Plan your crossing to give yourself the ability to turn and descend toward lower terrain quickly if necessary.

Rough Terrain

Heads up near or above abrupt changes of terrain such as cliffs or rugged areas. Dangerous turbulence can be expected, especially with high winds.

Box Canyons

Try to avoid flying up the middle of a canyon. It’s better to fly along one side or the other (preferably the downwind side) at sufficient altitude to be in a better position to execute a 180-degree turn. Allowing sufficient altitude for a descending 180-degree turn along with a turn into the wind (if possible) decreases actual turn radius across the ground. Use extra caution when mountain tops are obscured. Many accidents occur as a result of pilot’s turning up the wrong drainage, ending sadly in a box canyon. Monitor GPS (global positioning system) closely.

 

 

Additional Resources

Incident Management Situation Report (IMSR)
Incident Response Pocket Guide (IRPG), PMS 461
NWCG Standards for Helicopter Operations, PMS 510
RT-130, Wildland Fire Safety Training Annual Refresher (WFSTAR)
Interagency Standards for Fire and Fire Aviation Operations (Red Book)
Wildland Fire Lessons Learned Center

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