MOUNTAIN FLYING

Dr. Chip West

National Weather Service

TERRAIN

WEATHER-RELATED ACCIDENTS

0 50 100 150 200 250 300

NTSB Accident Data for 1995

Obscurations

Thunderstorms

Icing

High Density Altitude

Turbulence

Low Ceilings

Reduced Visibilities

Adverse Winds

Fatal Accidents Non-Fatal Accidents

Valley Fog and Stratus

• Diurnal Cycle

• Outside vs Valley Observations

• Season – Fall to Spring

Fog and Stratus

Preflight

.Observations – sources

.ASOS/AWOS limitations

.Valley fog/stratus

.Winds aloft

.Terrain effects on wind

.Frost

Observations

• Types– ASOS

– AWOS

• ASOS– “tunnel view

– “Clr” < 12,000ft.

– Cloud heights AGL

– Point visibility

Weather Takeoff Factors

• Altitude

• Temperature

• Humidity

• Wind direction

• Worse case combination?

Enroute –To Where?

• Always leave yourself the option of turning to lower terrain or back

• Forecast/observed cloud heights vs wind direction and weather patterns

• Terrain/wind induced turbulence

• Mountain wave

• Venturi effect

• Valley winds

• Icing

• Approaching/crossing ridges

• How to fly valleys

Terrain effects on wind

• Up to a doubling of reported speeds when flow channeled between peaks

• Up to a doubling of reported speed when forced over or around a peak, ex. Grandfather mountain.

• Diurnal effects

– Morning and evening winds

Terrain Effects on Wind

Doubling or more of speed around peaks or between peaks

Turbulence over Ridges

• < 20 kts = Light to Moderate

• 20 – 30 kts = Moderate to occasional Severe

• > 30 kts = Moderate or frequent Severe

Mountain Waves

MECHANICAL TURBULENCE

MOUNTAIN WAVEFAVORABLE CONDITIONS

-WINDS 25 KTS OR GREATER ACROSS-WINDS INCREASING WITH HEIGHT-STABLE ATMOSPHERE

STRATOSPHERE

TROPOPAUSE

TROPOSPHERE

WAVE LENGTH5 TO 30 MILES

LENTICULAR CLOUD

ROTOR

TURBULENCE

CAP CLOUD

WIND

70

60

50

40

30

20

10

0

100

0’s

OF

FE

ET

MOUNTAIN WAVE CLOUDS

CCSL

ACSL

ROTOR

RESPONSE ITEM

THE EFFECTS OF MOUNTAIN WAVES OFTEN

EXTEND FROM THE SURFACE TO

A. LESS THAN 5,000 FEET.

B. THE ALTITUDE OF THE MOUNTAIN

CREST.

C. SLIGHTLY ABOVE THE

TROPOPAUSE.

A. LESS THAN 5,000 FEET.

B. THE ALTITUDE OF THE MOUNTAIN

CREST.

C. SLIGHTLY ABOVE THE

TROPOPAUSE.

Crossing Ridges

How to fly valleys

DOWNSLOPE FLOW

WIND

WINDWARD

LEEWARD

FAVORABLE CONDITIONS

WINDS MODERATE OR GREATER ACROSS MOUNTAINS

RESPONSE ITEM

IN ORDER TO AVOID AN ACCIDENT BY RAPIDLY DESCENDING INTO THE MOUNTAIN THE PILOT SHOULD:

A. CLIMB 5,000 FEET OR MORE ABOVE CREST.

B. DECREASE SPEED AT CREST HEIGHT.

C. FLY AT CREST HEIGHT BUT STAY OUT OF DOWNDRAFT.

A. CLIMB 5,000 FEET OR MORE ABOVE CREST.

B. DECREASE SPEED AT CREST HEIGHT.

C. FLY AT CREST HEIGHT BUT STAY OUT OF DOWNDRAFT.

Mountain Winds

Anabatic Windor Updraft Wind

Sun’s heating

Valley WindDiurnal heating induces up valley wind

Mountain BreezeEvening cooling induces down valley or

drainage winds

WARM

COLD

FRONTSWIND SHEAR TURBULENCE

FRONTS

MECHANICAL TURBULENCE

STRONG GUSTY SURFACE WINDS

Arrival/Landing

• “Localized” weather

• Wind, effects of local terrain on

• Time of day

• Surrounding terrain

Safety Summary

•Always remain in a position so you can turn toward lowering terrain – 45 & down

•Never fly beyond the point of no return – 180, idle