Mountain Flying

Mountain flying is flying in and over high ground, from crossing a range on a cross-country flight to operating into an aerodrome in a valley or on a hillside. The wind is stronger and less predictable, the air on the lee side can sink faster than a light airplane can climb, the thin air of a high aerodrome lengthens the takeoff and weakens the climb, and cloud can hide the summits while the valleys stay clear.

None of these hazards is unique to mountains, but there they arrive together and close to the terrain. The weather is treated in more depth in mountain waves and local winds, the performance in density altitude.

Why mountains are different

Over mountains the margins that make flat-country flying forgiving shrink:

• Terrain. The ground can rise faster than the airplane climbs, and a valley that narrows can leave no room to turn. Mountain peaks and forests do not count as safe forced-landing areas in EASA Class B performance planning, so a single-engine airplane flown commercially over such terrain must cruise high enough to glide to a suitable area from any point on the route (see forced landing and ditching).
• Radio and navigation. VHF travels essentially by line of sight, so a station that is easy to work at altitude may be shielded by the high ground once the airplane is down in a valley; climbing restores contact. VOR signals reflected from terrain cause scalloping, a rapid cyclic wander of the course needle with no warning flag, and NDB bearings suffer from mountain effect.
• Licensing. In Europe, operations to and from surfaces designated as requiring it need a mountain rating under FCL.815. The FAA has no equivalent rating.

Wind, turbulence and mountain waves

Wind blowing across a ridge rises on the windward side, giving updrafts, and sinks on the lee side. Close to the lee slope the descending air brings downdrafts and mechanical turbulence, and in a light airplane the sink can exceed the maximum rate of climb. Where the flow is squeezed through a valley or over a crest it speeds up, and by the Venturi effect the pressure falls: the altimeter over-reads, as it also does in air colder than ISA, so the true altitude is lower than indicated just where the terrain clearance is least.

In stable air, a strong wind across a ridge can set up a mountain wave, a train of standing waves that stays almost fixed over the ground downwind. EASA texts give three conditions that must occur together: a wind within about 30° of perpendicular to the ridge, at least 15 kt at summit level and increasing with height, and a marked stable layer, an inversion or isothermal layer, near summit height. FAA figures vary: the AIM gives a wind of 15 kt or more crossing the range at an angle of at least 30°, while other FAA weather guidance uses about 25 kt at ridge level. Lenticular clouds in the wave crests, a cap cloud on the ridge and ragged rotor cloud beneath are the visible signs, but in dry air the waves form with no cloud at all.

The worst turbulence is in the rotor, a horizontal eddy beneath the wave crests at or below ridge height on the lee side. The most powerful rotor lies under the first crest, about one wavelength downwind of the ridge, at the heights where light airplanes fly circuits and approaches at lee-side aerodromes. In the descending part of a wave, downdrafts commonly reach 1,000 to 2,000 ft/min and are sustained over miles; FAA material notes that they can exceed 2,000 ft/min.

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Mountains also make their own winds. By day the sun heats the slopes and a light anabatic wind, about 5 kt in ATPL texts, blows upslope. At night the slopes cool and a katabatic wind drains cold air downslope, averaging about 10 kt and strongest over snow under a clear sky with a slack pressure gradient. FAA material calls the same pair the valley breeze and the mountain breeze. At a valley aerodrome a katabatic flow can bring an unexpected tailwind or crosswind around dawn. A gradient wind channeled along a valley is strengthened by the constriction, and air that has crossed a range and left its moisture as rain on the windward slopes arrives on the lee side warmer and drier, the föhn effect.

Mountain wave is a forecast and reported hazard. A SIGMET covers severe mountain wave, and an AIRMET covers moderate mountain wave for flights below FL100, or FL150 in mountainous areas. Under SERA, a pilot who meets severe mountain wave makes a special air-report to the ATS unit.

Density altitude and performance

Mountain aerodromes are high, and on a warm afternoon the air is thinner still. Density altitude, pressure altitude corrected for non-standard temperature, rises by roughly 120 ft for each degree Celsius above ISA. A normally aspirated piston engine loses roughly 3 percent of its sea-level power for each 1,000 ft of density altitude, the propeller is less efficient, and the wing needs a higher true airspeed for the same indicated airspeed. The takeoff run and landing roll lengthen and the climb weakens just where the terrain is close. Leaning as the flight manual directs recovers some of the lost power. In the United States, towers and flight service stations at airports 2,000 ft or higher broadcast "Check Density Altitude" when the temperature reaches a set level, but computing it remains the pilot's job.

The flight manual's indicated speeds still apply, but the true airspeed behind them is higher, and at a given bank angle the turn radius grows with the square of the true airspeed, so a turn in a valley needs more room than the same turn near sea level.

On a light twin, the single-engine service ceiling, where the climb rate on one engine falls to 50 ft/min, may lie below the minimum en route altitude on a hot day; after an engine failure the airplane then drifts down. Transport airplanes plan for it before departure. Under CAT.POL.A.215 the one-engine-inoperative net flight path must either keep a positive gradient at least 1,000 ft above all terrain within 5 NM either side of track or, during a drift-down, clear it by at least 2,000 ft all the way to an aerodrome where a landing can be made. 14 CFR 121.191 has the same two methods and the same 1,000 ft and 2,000 ft margins, with a corridor of 5 statute miles on each side of track. Operators publish drift-down and decompression escape routes over high ground, because chemical passenger oxygen lasts only about 12 to 22 minutes (see one engine inoperative en route performance).

High crossings in an unpressurized airplane also bring hypoxia into play. Under 14 CFR 91.211 the required crew use oxygen beyond 30 minutes above 12,500 ft cabin altitude, and always above 14,000 ft; where the pilot-in-command cannot determine how a lack of oxygen would affect those on board, EASA's NCO.OP.190 requires the flight crew to use it for any period over 30 minutes between 10,000 and 13,000 ft, and everyone on board above 13,000 ft (see hypoxia and hyperventilation).

Crossing ridges and flying valleys

Ridge crossing is planned on the ground: a strong wind across the ridge in stable air is warning enough, even without lenticular cloud. FAA guidance is to cross ridges 3,000 to 5,000 ft above them in strong winds and to approach at about 45° to the ridge, so that if a downdraft or severe turbulence is met, a turn away toward lower ground takes little time and room. Approaching from the lee side, into wind, the pilot should expect downdrafts and turbulence before reaching the crest, and should avoid flying below ridge level on the lee side, where the rotors are.

If a downdraft exceeds the airplane's climb performance, trying to outclimb it only costs airspeed while the airplane keeps descending. The answer is horizontal: turn toward lower terrain and out of the descending air, accepting the height loss, without descending into the rotor zone. In turbulence, slow to the turbulence penetration speed or the design maneuvering speed, hold the attitude and let the altitude vary.

Valley flying follows the low ground when cloud or performance rules out crossing the tops. It keeps the airplane below the cloud, but it trades away options: radio contact and navigation signals fade, the slopes on both sides restrict turns, and a valley that rises or narrows ahead under a lowering cloud base can trap the airplane. The FAA's AIM advises against entering confined areas without enough room to make a 180° turn should conditions require it. A mountain pass on a US sectional chart carries its elevation, which is neither a recommended route nor a clearance altitude.

Weather and visibility

Moist air forced up the windward slopes forms orographic cloud and rain while the lee side may be clear. Cloud whose base lies below the summits sits on the high ground as hill fog and closes a route over the tops, even when the valleys look flyable; pressing on beneath a lowering base into rising terrain is a classic VFR trap.

Mountain obscuration is the condition in which the high ground is hidden in cloud, fog or precipitation. ICAO AIRMETs include it for flights at low level, and in the United States it is forecast by G-AIRMET Sierra together with IFR conditions. An obscured pass is to be treated as closed to VFR flight, however good the weather in the valley below, and continued VFR flight into cloud in mountainous terrain is a leading cause of fatal accidents (see controlled flight into terrain).

On clear, calm nights, cold air drains downslope into the valleys, so radiation fog and frost form there first and clear last, and under a winter inversion a valley can stay fog-bound far longer. A strong inversion over a valley airport also leaves warmer air above the surface, which reduces thrust and climb performance after takeoff. Icing can be worse over high ground: stratocumulus, which normally gives light to moderate icing, can give severe icing over mountains. Over snow under an overcast sky, flat light removes shadows and contrast until the surface cannot be told from the sky, and pilots have flown into snow-covered slopes in clear air.

Route planning and minimum altitudes

Over high terrain a route along valleys, passes and airways and close to aerodromes keeps more options open than a direct line across the ridges. The rules raise the required margin to 2,000 ft over mountains:

For VFR flight over mountains at night, a sound planning minimum is at least 2,000 ft above the highest terrain, on a route along valleys or airways, raised for strong winds. On a US sectional the maximum elevation figure of each quadrangle is the highest terrain or obstacle plus allowances, rounded up; it is not a clearance altitude, and an airplane below it is not assured of clearing everything in the quadrangle (see minimum safe altitudes).

Mountain aerodromes

Mountain aerodromes range from airline airports in high valleys to short, steep strips on hillsides, known in the Alps as altiports, with sloping runways, one-way approaches and departures set by the terrain, high density altitudes and valley winds. The runway of the Megève altiport, in the French Alps, is 548 m long and climbs 133 ft, from 4,697 ft to 4,830 ft, in sections of 7%, 9.3% and 3.8%, an average of about 7.4%. Landing is compulsory on QFU 153°, uphill, and takeoff on QFU 333°, downhill, with a single departure sector. Airplanes arrive through a compulsory reporting point at 5,600 ft AMSL and overfly the aerodrome at 5,300 ft AMSL to check the runway and its snow cover. Unmarked cable-car cables lie near the circuits and the runway, and the aerodrome is reserved for airplane pilots holding the mountain qualification or a site authorization.

Slope changes the performance. An upslope lengthens the takeoff: where the flight manual gives no slope data, EASA Class B planning factors the takeoff distance by 1.05 for each 1 percent of upslope, about 1.10 for 2 percent, while FAA material notes that a weak performer at high density altitude can lose about 10 percent for each 1 percent. On landing an upslope helps, because the slope component of weight aids the deceleration, while a downslope lengthens the landing roll. A runway like Megève's is therefore landed uphill and taken off downhill.

Slope also misleads the eye. An upsloping runway looks as if the approach is steep, so a pilot unfamiliar with it feels high and tends to fly a low approach, risking an undershoot; a downsloping runway produces the opposite illusion (see visual illusions).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.