Operational Hazards

The clean-aircraft concept is absolute: frost, ice, snow and slush must not contaminate critical surfaces at take-off.

Why a thin deposit matters

Surface roughness as small as 0.40 mm, about 1/64 inch, can disturb airflow. Contamination reduces lift and manoeuvrability, increases drag and mass, blocks pitot, static or angle-of-attack sensors, obscures windscreens and affects undercarriage operation. There is no operationally insignificant layer of ice on a critical surface.

Cold-soaked wings

Fuel cooled during flight may keep the wing skin below freezing after landing, even when outside air temperature is above 0°C. Rain or high humidity can then produce clear ice. It may be hard to see, so a tactile inspection or an approved ground ice detection system may be required. A detached sheet can be ingested by an engine or strike the tail.

De-icing and anti-icing

Holdover time

Holdover time begins at the start of the final de-icing or anti-icing application. It is an operational estimate, not a guarantee. Fluid type and concentration, precipitation type and intensity, temperature, wind and aircraft skin temperature can shorten it. If the time expires or conditions worsen, the aircraft needs the prescribed pre-take-off contamination check and further treatment when necessary.

At night, an aircraft operating in expected or actual icing needs a means to illuminate or otherwise detect ice without glare or reflection that would impair the crew.

Wildlife, deliberate illumination and community-noise procedures create different hazards, but all require timely information and disciplined crew action.

Bird and wildlife strike risk

Aerodromes monitor, model, predict and warn about wildlife activity. Bali notes that shell crackers are an effective dispersal method, about 90 per cent of bird strikes occur below 500 m, and at a typical take-off speed of 135 kt a bird may begin moving only about two seconds before impact. Crews use warnings, lights and safe speed, avoid known concentrations when practicable and report strikes or hazardous concentrations promptly. Reports help identify species, location, height, phase of flight and damage, allowing the airport and DGCA system to manage recurring risk.

Laser illumination

A laser can cause glare, flash blindness, distraction and after-images, especially at night and during approach. The illuminated pilot looks away, avoids rubbing the eyes, uses instruments, transfers control if necessary and informs ATC with position, altitude, colour and direction of the beam. Affected crew obtain medical assessment when symptoms remain, and the occurrence is reported through the operator's safety system.

Noise-abatement limits

• No noise-abatement manoeuvre is made below 240 m, or 800 ft, above aerodrome elevation.
• The operator uses the same approved procedure for an aircraft type at all aerodromes.
• The procedure is not used when windshear warnings exist or a downburst is suspected.
• The maximum body angle is not exceeded, and a power reduction is not combined with a turn.
• Final landing configuration is established after the outer marker or 5 NM from touchdown, whichever is earlier.
• Noise-preferential routes avoid sensitive areas, but safety and ATC requirements remain controlling.

Noise is not the determining factor in runway selection when crosswind including gusts exceeds 15 kt, tailwind including gusts exceeds 5 kt, or the runway is not clear and dry. Local restrictions may prohibit reverse thrust above idle during stated periods, particularly at night.

Fire needs fuel, heat and oxygen. Extinguishment removes at least one side of that triangle, but the correct agent depends on what is burning.

Portable extinguishers

A portable bottle may discharge for only 8 to 25 seconds, so the crew gets close enough for an effective burst while maintaining a safe escape path. Bali describes halon as about three times as effective as the same mass of carbon dioxide. When halon knocks down a class A fire, the material still needs cooling with a suitable nonalcoholic liquid. In a confined area the crew uses protective breathing equipment. At least one suitable halon-type extinguisher is available on the flight deck, with appropriate equipment for galley, cabin and cargo risks.

An overheated wheel and a loss of cabin pressure can both become time-critical emergencies. The crew's first action protects life, then stabilises the aircraft and coordinates help.

Overheated brakes

A rejected take-off or heavy landing may overheat the brakes, causing reduced braking, fire, tyre deflation or burst. Brake failure can produce a runway or taxiway excursion or a collision, and high workload may delay recognition. The crew informs ATC, requests rescue and fire services, follows the aircraft checklist and avoids placing people in the wheel's danger arc. If fire threatens occupants, an emergency evacuation may be necessary.

Loss of pressurisation

Decompression may follow structural failure, pressurisation-system failure or deliberate action. A rapid decompression takes seconds; a decompression in less than about half a second is classed as explosive. Dust, debris or cabin fog, a sharp temperature and humidity change, noise and pressure discomfort may occur. Crew can be startled or temporarily dazed. Slow decompression offers more reaction time but can be harder to notice.

Structural damage may bring buffeting, extreme cold, flying debris and an ejection risk. Seat belts limit injury. The priorities are oxygen, aircraft control, emergency descent, communication and assessment of damage.

A microburst can change an apparently useful headwind into a severe downdraft and tailwind before an aircraft has enough height or energy to recover normally.

Recognition criteria

Significant nonconvective low-level shear includes a vector wind change greater than 25 kt within 500 ft AGL, greater than 40 kt within 1,000 ft, or greater than 50 kt within 1,500 ft. A pilot report of an indicated-airspeed gain or loss of at least 20 kt within 1,500 ft is also significant.

Microburst structure

A microburst is a small, intense downdraft that spreads rapidly outward near the surface. Bali gives a diameter of about 1 NM at 2,000 ft AGL and a surface outflow about 2 to 2.5 NM across. Vertical velocity can reach 6,000 ft per minute and horizontal outflow 45 kt, producing as much as 90 kt of shear. It can be wet or dry, with virga as a warning. One cell usually lasts no more than 15 minutes and reaches maximum winds for about two to four minutes, while a line of cells can keep the hazard active for an hour.

Encounter sequence and action

1. On entry, increasing headwind raises indicated airspeed and performance.
2. The central downdraft forces the flight path downward.
3. On exit, a tailwind sharply reduces indicated airspeed and climb margin.

The safest response is avoidance. If an encounter occurs, follow the aircraft windshear escape guidance, apply maximum approved thrust, set the commanded escape pitch or maximum usable angle of attack, minimise configuration change and warn ATC and following aircraft.

Wingtip vortices are the rotating wake of lift. They are strongest when the generating aircraft is heavy, clean and slow.

Formation and movement

Higher pressure below the wing curls around the tip toward lower pressure above it. Viewed from behind, the left vortex rotates clockwise and the right vortex counterclockwise. The pair sinks below the flight path, commonly remaining within about 1,000 ft below it, and can spread laterally for several miles. Close to the ground the vortices move outward. Wind can carry one onto a runway or hold an upwind vortex near it.

Wake categories in Bali

Avoidance technique

• Remain above and upwind of the preceding aircraft's flight path.
• For departure behind a larger aircraft, lift off before its rotation point and climb above its path.
• For landing, remain above its glide path and touch down beyond its touchdown point.
• Allow extra care in calm wind and a light quartering tailwind, when a vortex may remain on or drift across the runway.
• On parallel runways, consider crosswind drift from the upwind runway.

Time-based and surveillance-based wake minima protect a following aircraft where its track and level can place it in a stronger aircraft's descending wake.

Arrival time minima

Departure time minima

Surveillance distance minima

The distance minima apply when a following aircraft is directly behind at the same level or less than 1,000 ft below, on the same runway, on parallel runways less than 760 m apart, or on a crossing track behind the leader. The larger applicable normal radar minimum always takes precedence. Time spacing can replace distance spacing on final where prescribed, including conditions with strong headwind.

Fuel dumping and an abnormal landing are coordinated procedures. The commander communicates the plan, protects people and traffic, and preserves the safest attainable aircraft state.

Forced landing, precautionary landing and ditching

Fuel-jettison area

When an aircraft must reduce mass, ATC and the crew agree on an area clear of cities and towns, preferably over water and away from thunderstorms. Bali states a height of at least 1,800 m, or 6,000 ft. The crew advises how long the jettison will take, monitors an agreed frequency and reports completion. Other aircraft maintain radio silence on that frequency unless safety requires transmission.

Protected airspace

Other traffic stays at least 19 km, or 10 NM, horizontally from the jettisoning aircraft and not behind it. Traffic behind within 15 minutes or 93 km, or 50 NM, remains at least 300 m, or 1,000 ft, above or 900 m, or 3,000 ft, below. The protected area extends 10 NM either side, 10 NM ahead and to 50 NM or 15 minutes behind, including turns.

Water, slush, snow or ice can reduce tyre friction, interfere with anti-skid performance and make the available runway very different from the dry-runway calculation.

Surface descriptions

Dynamic hydroplaning

When standing water is deep enough relative to tyre tread and speed, hydrodynamic pressure can lift the tyre from the pavement. Braking and directional control may become almost ineffective. Water depth, tyre pressure and groundspeed influence the threshold. Anti-skid minimises locked-wheel skidding but cannot create friction when the tyre is riding on water.

Operational response

Use the reported runway condition and approved performance data, make a stabilised touchdown in the correct zone, lower the nosewheel positively, apply aerodynamic braking and approved wheel braking and reverse procedures, and avoid abrupt control inputs. Bali states that when specific wet-runway landing data are unavailable, the landing distance available must be at least 115 per cent of the required dry-runway landing distance.

Dangerous goods are articles or substances capable of posing a hazard to health, safety, property or the environment. ICAO Annex 18 and the Technical Instructions organise them by hazard and control every step from acceptance to emergency response.

The nine classes

Operator provisions

An operator carrying dangerous goods needs approval, trained personnel and procedures for acceptance, document checking, package inspection, marking and labelling, loading, segregation of incompatible goods, securing, damage and leakage checks, storage, information to the pilot-in-command and emergency reporting. Packages showing damage or leakage are not loaded. Contamination is removed and an affected aircraft or load position is inspected before return to service.

Passenger and crew provisions

Dangerous goods are not carried in the passenger cabin or on the flight deck unless the Technical Instructions expressly permit them. Normal exceptions cover specified personal medicinal or toiletry articles and limited operational items under stated conditions. Spare batteries are protected against short circuit and carried where required by the current instructions. Mobility aids, medical oxygen, dry ice and other special items may require operator approval, quantity limits or notification. Undeclared dangerous goods are refused and reported.

Goods forbidden in all normal circumstances

Articles identified by the Technical Instructions as forbidden under normal circumstances, and infected live animals, are not carried unless a specific exemption permits the operation. The commander must receive written information about dangerous goods on board before departure so that emergency decisions and information to rescue services are accurate.

Volcanic ash and terrain alerts demand immediate operational decisions. Each warning system has a different source, but none removes the pilot's responsibility to protect the flight path.

Volcanic ash

Ash can abrade windscreens and leading edges, block probes and filters, contaminate cabin air and melt inside a turbine engine, causing compressor damage or flameout. Weather radar may not display a dry ash cloud reliably. Avoid the reported cloud and visible ash. If an encounter occurs, exit by the shortest safe route and apply the aircraft checklist, which may call for reduced thrust, ignition and engine anti-ice settings. Report position, level, time and observed effects to ATC.

VAAC, advisory and SIGMET

A Volcanic Ash Advisory Centre, or VAAC, analyses observations, satellite data and forecasts and issues advisory information for its area. Meteorological watch offices use that information to issue a SIGMET for volcanic ash affecting a flight information region. Operators combine the advisory, SIGMET, NOTAM and other official information for route planning and re-planning.

The colour code describes volcanic activity for aviation users. It is not a clearance to enter ash and does not replace the mapped ash forecast.

Minimum safe altitude warning

MSAW is a ground-based ATS safety net. It alerts the controller when a surveillance-tracked aircraft is, or is predicted to be, close to a minimum safe altitude. The controller immediately issues a low-altitude or terrain warning and, when appropriate, a suggested corrective action. The crew checks altitude and flight path at once.

GPWS and terrain awareness

Ground proximity warning and terrain awareness systems are airborne safety nets. Cautions prompt immediate assessment and the published crew response. A warning such as PULL UP or TERRAIN requires the operator's prescribed escape manoeuvre without delay unless the crew has positive visual assurance that no hazard exists. An emergency response takes priority over an ATC clearance; the crew informs ATC as soon as workload permits.