Adverse and Potentially Hazardous Atmospheric Conditions

Adverse and Potentially Hazardous Atmospheric Conditions

This chapter contains a compilation of hazardous atmospheric conditions and recommended practices and procedures for operating in and/or avoiding those conditions associated with:

• Thunderstorms;
• Icing conditions in flight;
• Turbulence;
• Windshear;
• Jetstream;
• Volcanic Ash Clouds;
• Heavy Precipitation;
• Sandstorms;
• Mountain Waves;
• Significant Temperature Inversion;
• Operations on Slippery Surfaces
• Tropical Storms (Typhoons, Hurricanes, Cyclones).

Thunderstorms

General

There is no useful correlation between the external visual appearance of thunderstorms and their severity. Knowledge and weather radar have modified attitudes toward thunderstorms, but one rule continues to be true:

Any thunderstorm should be considered hazardous!

Weather Information

Meteorological observations/forecasts messages or charts contain thunderstorm and associated hazards information. But, when thunderstorms are, or are expected to be sufficiently widespread to make their avoidance difficult (e.g. a line of thunderstorms associated with a front or squall line or extensive high-level thunderstorms), the Meteorological Service issues warnings in the form of SIGMET messages of “Active Thunderstorm Area”. In addition, pilots are required to send a special air report (PIREP) when conditions are encountered which are likely to affect the safety of the aeroplane. Such a report would be the basis of a SIGMET warning.

The Meteorological Service generally does not issue SIGMET messages in relation to isolated thunderstorm activity and the absence of SIGMET warnings does not therefore necessarily indicate the absence of thunderstorms.

Thunderstorm Hazards

Thunderstorms incorporate every weather hazard to aviation into one vicious package.

The most important hazards are:
• Turbulence: Potentially hazardous turbulence is present in all thunderstorms. Strongest turbulence within the cloud occurs with shear between updrafts and downdrafts. Outside the cloud, shear turbulence has been encountered several thousand feet above and 20 NM laterally from a severe storm. A low-level turbulent area is the shear zone associated with the gust front.

Often, a “roll cloud” on the leading edge of a storm marks the top of the eddies in this shear and it signifies an extremely turbulent zone. Gust fronts often move far ahead (up to 15 NM) of associated precipitation. The gust front causes a rapid and sometimes drastic change in surface wind ahead of an approaching storm. It is almost impossible to hold a constant altitude in a thunderstorm, and manoeuvring in an attempt to do so produces greatly increased stress on the aeroplane. It is understandable that the speed of the aeroplane determines the rate of turbulence encounters. Stresses are least if the aeroplane is held in a constant attitude and allowed to ‘ride the waves’ (refer to OM Part B for guidance on flight in severe turbulence).

• Icing: Supercooled water freezes on impact with an aeroplane. Clear icing can occur at any altitude above the freezing level; but at high levels, icing from smaller droplets may be rime or mixed rime and clear. The abundance supercooled water droplets make clear icing very rapid between 0ºC and -15ºC.

• Hail: Hail competes with turbulence as the greatest thunderstorm hazard to aeroplanes. Supercooled drops above the freezing level begin to freeze. Once a drop has frozen, other drops latch on and freeze to it, so the hailstone grows. Large hail occurs with severe thunderstorms with strong updrafts that have built to great heights. Eventually, the hailstones fall, possibly some distance from the storm core. Hail may be encountered in the clear air several miles from dark thunderstorm clouds.

Low Ceiling and Visibility: Generally, visibility is near zero within a thunderstorm cloud. The hazards and restrictions created by low ceiling and visibility are increased when associated with the other thunderstorm hazards.

• Effect on Altimeters: Pressure usually falls rapidly with the approach of a thunderstorm, and then rises sharply with the onset of the first gust and arrival of the cold downdraft and heavy rain showers, failing back to normal as the storm moves on. This cycle of pressure change may occur in 15 minutes. If the pilot does not receive a corrected altimeter setting, the altimeter may be more than 1.000 ft in error.

• Lightning: A lightning strike can puncture the skin of an aeroplane. Lightning has been suspected of igniting fuel vapours causing the explosion; however, serious accidents due to lightning strikes are extremely rare. Nearby lightning can blind the pilot rendering him momentarily unable to navigate either by instrument or by visual reference. Lightning can also induce permanent errors in the magnetic compass and lightning discharges, even distant ones, can disrupt radio communications on low and medium frequencies. The Lightning intensity and frequency have no simple relationship to other storm parameters. But, as a rule, severe storms have a high frequency of lightning. Each lightning strike must be recorded in Aircraft Technical Log.

• Engine Water Ingestion: Jet engines have a limit on the amount of water they can ingest. Updrafts are present in many thunderstorms, particularly those in the development stages. If the updraft velocity in the thunderstorms approaches or exceeds the terminal velocity of the falling raindrops, very high concentrations of water may occur. It is possible that these concentrations can be the excess of the quantity of water engines are designed to ingest. Therefore, severe thunderstorms may contain areas of high water concentration which could result in flame-out and/or structural failure of one or more engines.

Avoiding Thunderstorms

Never regard a thunderstorm harmless. Avoiding thunderstorms is the best policy. General rules are:

• Don’t land or take-off in the face of an approaching thunderstorm. Turbulence wind reversal or windshear could cause loss of control.
• Don’t attempt to fly under a thunderstorm even if you can see through to the other side. Turbulence and wind shear under the storm could be disastrous.
• Don’t fly without airborne radar into a cloud mass containing scattered embedded thunderstorms. Scattered thunderstorms not embedded usually can be visually circumnavigated.
• Don’t trust the visual appearance to be a reliable indicator of the turbulence inside a thunderstorm. Do avoid by at least 20 NM any thunderstorm identified as severe or giving an intense radar echo. This is especially true under the anvil of large cumulonimbus.
• Do circumnavigate the entire area if the area has 6/10 thunderstorm coverage.
• Do remember that vivid and frequent lightning indicates the probability of a severe thunderstorm.
• Do regard as extremely hazardous any thunderstorm with tops 35.000 ft or higher whether the top is visually sighted or determined by radar.

Departure and Arrival: When significant thunderstorm activity is approaching within 15 NM of the airport, the Commander should consider conducting the departure or arrival from the different direction or delaying the take-off or landing. Use all available information for this judgement, including PIREPs, ground radar, aeroplane radar, tower-reported winds, and visual observations. In the terminal area, thunderstorms should be avoided by no less than 3 NM. Many ATC radars are specifically designed to reduce or exclude returns from ‘weather’ and in these cases, little or no assistance can be given by ATC. It is recommended that any guidance given by ATC should be used in conjunction with the aeroplane own weather radar, in order to guard against possible inaccuracies in the ground radars interpretation of the relative severity of different parts of a storm area. Any discrepancies should be reported to ATC. Gust fronts in advance of a thunderstorm frequently contain high winds and strong vertical and horizontal wind shears, capable of causing an upset near the ground. A gust front can affect an approach corridor or runway without affecting other areas of the airport. Under such conditions, tower reported winds and the altimeter setting could be misleading.

Microbursts: 2 NM or less in diameter, microbursts are violent short-lived descending columns of air capable of producing horizontal winds sometimes exceeding 60 kts within 150 ft of the ground. Microbursts commonly last one to five minutes and may emanate from high-based cumulus clouds accompanied by little or no precipitation, or may be associated with large cumulonimbus build-ups and be accompanied by heavy rainfall.

Because of their relatively small diameter, airport anemometers and low-level wind shear alert systems may not sense this phenomenon in time to provide an adequate warning of nearby microburst activity.

Overflying: Avoid overflying thunderstorms unless a minimum of 5.000 ft clearance above the storm top is ensured. When possible, detour between the storm cells of a squall line rather than directly above them. Keep the radar antenna tilted down during overflight to properly assess the most severe cells, which may be masked by clouds formations.

Lateral Avoidance: At altitudes above the freezing level, super-cooled rain and hail may indicate as only weak radar echoes, which can mask extreme thunderstorm intensity.

Radar echoes associated with thunderstorms should be avoided by the following minimum distances:

Flight near Thunderstorms

If flight closer than the minimum recommended distances is unavoidable, observe the following precaution:

• When it is necessary to fly parallel to a line of cells, the safest path is on the upwind side (the side away from the direction of storm travel). Although severe turbulence and hail can be encountered in any direction outside a thunderstorm, strong drafts and hail are more often encountered outside the body of the cell on the downwind side.
• Avoid flight under the anvil. The greatest possibility of encountering hail is downwind of the cell, where hail falls from the anvil or is tossed out from the side of the storm. Hail has been encountered as much as 20 NM downwind from large thunderstorms.
• Avoid Cirrus and Cirrostratus layers downwind from the storm tops. Such layer may be formed by cumulonimbus tops and may contain hail, even though the radar scope shows little or no return echoes.
• If ATC requirements make the flight into unsafe conditions imminent, the Commander should request a change of routeing and if necessary use his emergency authority to avoid the severe weather conditions.
• Any flight in the vicinity of thunderstorms carries the risk of a sudden onset of moderate or severe turbulence.

Thunderstorm Penetration

If thunderstorm penetration is unavoidable, the following guidelines will reduce the possibility of entering the worst areas of turbulence and hail:

• Prepare the aeroplane for severe turbulence - refer to OM Part B;
• Use the radar to determine the areas of least precipitation. Select a course affording a relatively straight path through the storm. Echoes appearing hooked, finger-like, or scalloped indicate areas of extreme turbulence, hail and possibly tornadoes, and must be avoided;
• Penetrate perpendicular to the thunderstorm line; if not possible maintain the original heading. Once inside the cell, continue ahead, a straight course through the storm most likely get the aeroplane out of the hazards most quickly. The likelihood of an upset is greatly increased when a turn is attempted in severe turbulence and turning manoeuvres increase the stress on the aeroplane;
• Pressure changes may be encountered in strong drafts and may conduct to an altitude error of 1.000 ft;
• Gyro-stabilised instruments supply the only accurate flight instrument indications;
• Avoid level near the 0ºC isotherm. The greatest probability of severe turbulence and lightning strikes exist near the freezing level;
• Generally, the altitudes between 10.000 ft and 20.000 ft encompass the most severe turbulence, hail, and icing conditions, although violent weather may be encountered at all level inside and outside an active thunderstorm;
• Due to very high concentration of water, massive water ingestion can occur which could result in engine flameout and/or structural failure of one or more engines. Changes in thrust should be minimised.

Operational Procedures

Whenever flying through or near to a thunderstorm, the following procedures and techniques are recommended:

• Approaching the thunderstorm area make sure that crew members' safety belts are firmly fastened and secure any loose articles;
• Switch ON the Seat Belt signs and ensure that all passengers are securely strapped in and that loose equipment (e.g. cabin trolleys and galley containers) are firmly secured;
• Pilots should remember that the effect of turbulence is normally worse in the rear of the aeroplane that on the flight deck;
• One pilot should fly the aeroplane and control aeroplane attitude regardless of all else and the other monitor the flight instruments continuously;
• Height for penetration must be selected bearing in mind the importance of insuring adequate terrain clearance. Due to turbulence, wind shear, local pressure variations the maintenance of a safe flight path can be difficult;
• The recommended speed for flight in turbulence must be observed (refer to OM Part B);
• The autopilot should remain engaged. The autopilot is likely to produce lower structural loads and smaller oscillations than would result from a manual flight. The auto-thrust should be disconnected to avoid unnecessary and frequent thrust variations;
• Check the operation of all anti-icing equipment and operate all these systems in accordance with OM Part B instructions. Icing can be very rapid at any altitude;
• Turn the cockpit lighting fully ON to minimise the blinding effect of lightning;
• Continue monitoring the weather radar in order to pick out the safest path. Tilt the antenna up and down occasionally to detect thunderstorm activity at altitudes other than that being flown.