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Mountain Waves

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Article Information
Category: Weather Weather
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary
WX
Tag(s) Weather Phenomena,
Turbulence

Lee Waves or Standing Waves

Definition

Mountain Waves is defined as oscillations to the lee side (downwind) of high ground resulting from the disturbance in the horizontal air flow caused by the high ground.

Description

The wavelength and amplitude of the oscillations depends on many factors including the height of the high ground relative to surrounding terrain, the wind speed and the instability of the atmosphere.

Formation of mountain waves can occur in the following conditions:

  • Wind direction within 30 degrees of the perpendicular to the ridge of high ground and no change in direction over a significant height band.
  • Wind speeds at the crest of the ridge in excess of 15 kts27.78 km/h
    7.71 m/s
    , increasing with height.
  • Stable air above the crest of the ridge with less stable air above and a stable layer below the ridge.
Leewaves
Mountain waves

Vertical currents within the oscillations can reach 2,000 ft/min10.16 m/s. The combination of these strong vertical currents and surface friction may cause rotors[1] to form beneath the mountain waves causing severe turbulence.

Effects

Mountain Waves are associated with severe turbulence, strong vertical currents, and icing.

  • Loss of Control and / or Level Bust. The vertical currents in the waves can make it difficult for an aircraft to maintain en route altitude leading to level busts and can cause significant fluctuations in airspeed potentially leading, in extremis, to loss of control. Loss of Control can also occur near to the ground prior to landing or after take off with a risk of terrain contact or a hard landing if crew corrective response to a downdraft is not prompt.
  • Turbulence. Aircraft can suffer structural damage as a result of encountering severe clear air turbulence. In extreme cases this can lead to the break up of the aircraft. In even moderate turbulence, damage can occur to fittings within the aircraft especially as a result of collision with unrestrained items of cargo or passenger luggage. If caught unaware, passengers and crew walking around the aircraft cabin can be injured.
  • Icing. Severe icing can be experienced within the clouds associated with the wave peaks.

Defences

Lenticular Clouds
Lenticular Clouds (lens shaped clouds)
Lenticular clouds over Luino, Italy, phoptographed on 17 March 2008. Source: Jacob Kollegger, RMetS.
  • Awareness.
    • When approaching a mountain ridge, it is advantageous, if heading upwind towards it, to cross at an angle of around 30 - 45 degrees in order to allow an escape should downdrafts prove excessive.
    • In the Alps regions, particularly in the Zurich – Milano regions, a general rule of thumb that a QNH difference of more than 5 – 8 mb between LSZH and LIMC, for example, or between north and south of the Alps, will provide for significant mountain wave activity over the Alps. A higher QNH in Zurich will result in mountain waves south of the Alps, for example.
    • If significant mountain wave activity is expected, as a rule of thumb and if possible plan a flight at least 5000 – 8000 feet above the highest elevation along your route.
  • Forecasting. Local knowledge of the conditions which tend to cause the formation of mountain waves enables forecasting of potential wave propagation.
  • Cloud Formation. Lenticular Clouds (lens shaped clouds) can form in the crest of the mountain waves if the air is moist. Roll Clouds can also occur in the rotors below the waves if the air is moist. These clouds are a good indication of the presence of mountain waves but, if the air is dry, there may not be any cloud to see. Windward of the mountains IMC conditions may likely be present, whereas due to the “Foehn Effect” VMC conditions are generally expected to the leeward.
  • Restraint Systems. Passengers and flight crew should routinely wear their seat belts / harnesses when seated to provide protection against unexpected turbulence.

Scenarios

An aircraft tracking perpendicularly across, or downwind of, a mountain range or a significant mountain ridge experiences a sudden loss of altitude followed by a significant and sudden reduction in airspeed during severe turbulence. Regaining the desired flght path may be difficult, for a relatively short period, until the wave is exited.

Solutions

  • Airspeed. Reducing the aircraft speed reduces the risk of structural damage and reduces vibration making instruments easier to read in turbulence BUT beware the effect of vertical currents on airspeed and the risk of stalling the aircraft.
  • Strap in. Aircraft commanders should ensure that if conditions in which there is a possibility of encountering mountain wave turbulence are envisaged en route that not only are seat belt signs selected on but that all cabin crew are instructed to cease service and secure their equipment and themselves in good time.
  • Inform ATC. Notify ATC of mountain wave activity.

Related Articles

Accidents & Incidents

The following reports include reference to Mountain Wave activity:

  • SW4, Sanikiluaq Nunavut Canada, 2012 (On 22 December 2012, the crew of a Swearingen SA227 attempting a landing, following an unstabilised non-precision approach at Sanikiluaq at night with questionable alternate availability in marginal weather conditions, ignored GPWS PULL UP Warnings, then failed in their attempt to transition into a low go around and the aircraft crashed into terrain beyond the runway. One occupant – an unrestrained infant – was killed and the aircraft was destroyed. The Investigation faulted crew performance, the operator and the regulator and reiterated that lap-held infants were vulnerable in crash impacts.)
  • CL60, Birmingham UK, 2002 (On 4 January 2002, the crew of US-operated Bombardier Challenger lost control of their aircraft shortly after taking off from Birmingham and after one wing touched the ground, it rolled inverted, crashed and caught fire within the airport perimeter and all five occupants died. The Investigation found that the cause of the accident was failure to remove frost from the wings which reduced the wing stall angle of attack below that at which the stall protection system was effective. It was considered that the combined effects of non-prescription drug, jet lag and fatigue may have impaired crew performance.)
  • DHC6, en-route, Arghakhanchi Western Nepal, 2014 (On 16 February 2014 a Nepal Airlines DHC6 attempting a diversion on a VFR flight which had encountered adverse weather impacted terrain at an altitude of over 7000 feet in a mountainous area after intentionally entering cloud following a decision to divert due to weather incompatible with VFR. The aircraft was destroyed and all 18 occupants were killed. The Investigation attributed the accident to loss of situational awareness by the aircraft commander and inadequate crew co-operation in responding to the prevailing weather conditions.)
  • AS32, en-route, North Sea UK, 2002 (On 28th February 2002, an Aerospatiale AS332L Super Puma helicopter en route approximately 70 nm northeast of Scatsa, Shetland Islands was in the vicinity of a storm cell when a waterspout was observed about a mile abeam. Soon afterwards, violent pitch, roll and yaw with significant negative and positive ‘g’ occurred. Recovery to normal flight was achieved after 15 seconds and after a control check, the flight was completed. After flight, all five tail rotor blades and tail pylon damage were discovered. It was established that this serious damage was the result of contact between the blades and the pylon.)
  • B738, vicinity Eindhoven Netherlands, 2013 (On 31 May 2013, a Boeing 737-800 (EI-ENL) being operated by Ryanair on a scheduled international passenger flight from Palma del Mallorca to Eindhoven as FR3531 was established on the ILS LOC in day IMC with the AP and A/T engaged and APP mode selected but above the GS, when the aircraft suddenly pitched up and stick shaker activation occurred. After a sudden loss of airspeed, the crew recovered control manually and the subsequent approach was completed without further event.)
  • AT43, en-route, Folgefonna Norway, 2005 (On 14 September 2005, an ATR 42-320 operated by Coast Air AS experienced a continuous build up of ice in the climb, despite the activation of de-icing systems aircraft entered an uncontrolled roll and lost 1500ft in altitude. The crew initiated recovery actions, the aircraft was stabilised, and the flight continued without further event.)
  • A320, Hamburg Germany, 2008 (On 1 March 2008 an Airbus A320 being operated by Lufthansa on a scheduled passenger flight from Munich to Hamburg experienced high and variable wind velocity on short finals in good daylight visibility and during the attempt at landing on runway 23 with a strong crosswind component from the right, a bounced contact of the left main landing gear with the runway was followed by a left wing down attitude which resulted in the left wing tip touching the ground. A rejected landing was then flown and after radar vectoring, a second approach to runway 33 was made to a successful landing. No aircraft occupants were injured but the aircraft left wing tip was found to have been damaged by the runway contact. The track of the aircraft and spot wind velocities given by ATC at key points are shown on the illustration below.)
  • B738, vicinity Faro Portugal, 2011 (On 24 October 2011, the crew of a Ryanair Boeing 737-800 operating the first flight after an unexpectedly severe overnight storm found that after take off, an extremely large amount of rudder trim was required to fly ahead. Following an uneventful return to land, previously undetected damage to the rudder assembly was found which was attributed to the effects of the storm. It was found that pre flight checks required at the time could not have detected the damage and noted that the wind speeds which occurred were much higher than those anticipated by the applicable certification requirements.)
  • D228, vicinity Bodø Norway, 2003 (On 4 December 2003, the crew of a Dornier 228 approaching Bodø lost control of their aircraft after a lightning strike which temporarily blinded both pilots and damaged the aircraft such that the elevator was uncontrollable. After regaining partial pitch control using pitch trim, a second attempt at a landing resulted in a semi-controlled crash which seriously injured both pilots and damaged the aircraft beyond repair. The Investigation concluded that the energy in the lightning had probably exceeded certification resilience requirements and that up to 30% of the bonding wiring in the tail may have been defective before lightning struck.)
  • A109, vicinity London Heliport London UK, 2013 (On 16 January 2013, an Augusta 109E helicopter positioning by day on an implied (due to adverse weather conditions) SVFR clearance collided with a crane attached to a tall building under construction. It and associated debris fell to street level and the pilot and a pedestrian were killed and several others on the ground injured. It was concluded that the pilot had not seen the crane or seen it too late to avoid whilst flying by visual reference in conditions which had become increasingly challenging. The Investigation recommended improvements in the regulatory context in which the accident had occurred.)
  • B744, en-route, Alaska USA, 1989 (On 15th December 1989, a KLM Boeing 747 encountered a Volcanic Ash cloud over Alaska, USA. the ingestion of ash led to compressor stall of all engines; the engines were subsequently relighted successfully and the aircraft landed safely.)
  • GLF3, Biggin Hill UK, 2014 (On 24 November 2014, the crew of a privately-operated Gulfstream III carrying five passengers inadvertently commenced take off at night in poor visibility when aligned with the runway edge instead of the runway centreline. When the aircraft partially exited the paved surface, the take-off was rejected but not before the aircraft had sustained substantial damage which put it beyond economic repair. The Investigation found that chart and AIP information on the taxiway/runway transition made when lining up was conducive to error and that environmental cues, indicating the aircraft was in the wrong place to begin take-off, were weak.)

"Mountain Waves" is not in the list of possible values (In Flight Airframe Icing, In Flight Icing - Piston Engine, In Flight Icing - Turbine Engine, CAT encounter, En route In-cloud air turbulence, Hail damage, Volcanic Ash Effects, Fog, In Cloud on Visual Clearance, Precipitation-limited IFV, Strong Surface Winds, Lightning Damage, Low Level Windshear, In Flight Icing - Turboprop Engine, In Flight Icing - Jet Engine, Sand/Dust limited Forward Visibility, Mountain Wave/Rotor Conditions, Triggered Lightning Strike, Layer Cloud Airframe Icing) for this property.

Notes

  1. ^ Rotor: a turbulent horizontal vortex generated around the "troughs" of mountain wave activity (see diagram above).