Thursday, March 22, 2012

Types of Fire Extinguishants

Methyl Bromide
This extinguisher boils at 4.6°C and was used for the protection of power plants in older aircraft.  It is toxic and should not be used in confined spaces, flight crew compartments or passenger cabins.  This extinguisher is no longer listed.  Existing bottles may be maintained but refills will not be made with Methyl Bromide.

Bromochlorodifluoromethane (BCF)

This semi-toxic extinguisher is particular effective against electrical and flammable liquid fires.  It is used in power plant systems, and for the protection of auxiliary power units in some aircraft.  It is also used in certain types of portable extinguisher.  It becomes gaseous at normal temperatures and condenses to liquid at -4°C (25°F), and can be stored and discharged at moderate pressures.  It has little or no corrosive effect, although halogen acids will be formed if its products which have been decomposed by fire comes into contact with water, eg. condensation caused by fire.  In contact with fire, BCF volatilizes instantly, giving rapid flame extinction with little or no harmful effect on metallic, wooden, plastic or fabric materials.
Also known as Halon 1211.

Bromotrifluoromethane (BTM)

This semi-toxic extinguishant is used for the protection of power plant and APUs.  It is also widely used in cargo compartment fire suppression systems of some types of aircraft.
Also known as Halon 1301.  It has a boiling point of -58°C.

Thursday, March 1, 2012

Aircraft Towing

On aircraft with a nose-wheel landing gear, a steering arm should be fitted to the nose wheel to guide the aircraft Light aircraft can be moved and guided, by hand or by a tug.  

Special attention should be paid to the following:
  • Force should not be applied to the thin trailing or rear edges of wings or control surfaces such as ailerons or elevators.
  • Generally speaking it is better to push an aircraft backwards rather than forwards, because the leading edges of the wings and tailplane are stronger than the trailing edges.
  • The struts, which support the undercarriage on some aircraft, are suitable for pushing the aircraft as these are the strength parts of the aircraft.
  • The flat of the hands should be used when pushing, so as to spread the load over the largest area.
  • When pushing on struts, the force should be applied as near to the end fittings as possible.
  • A propeller must never be used to push or pull the aircraft, as the engine should always be regarded as 'live' and a propeller may kick if it is turned.
On aircraft with a steering nose wheel connected to the rudder pedals, care must be taken not to exceed the turning limits. Normally the maximum limits are marked on nose wheel doors as “NO TOW”

On this type of aircraft it is also important that the rudder controls are not locked during the towing operations.  This is because the rudder controls and the nose-wheel steering mechanism are interconnected and the excessive movements could damage the mechanical linkages.
When towing aircraft by tow bar and tug special care should be given to below facts,
The correct tow-bar should be connected between the towing attachment at the base of the nose undercarriage leg and the tug.
  • A person familiar with the aircraft brake system should be seated in the cockpit/cabin to operate the brakes in an emergency.  The brakes should not normally be applied unless the aircraft is stationary.
  • Once the tow-bar is connected, the brakes, and if fitted, the rudder lock, may be released and the aircraft towed at a safe speed.  A safe speed is considered to be walking speed.
  • A close watch should be kept on the wing tips and tail, particularly in confined spaces, to ensure that they do not come into contact with other stationary or moving object.
  • In circumstances where the ground over which the aircraft has to be towed is either boggy or very uneven, the strain imposed on the nose undercarriage may be excessive and it may be necessary to tow the aircraft by means of bridles attached to each main undercarriage.  If towing attachments are not provided on the main undercarriage legs, ropes should be passed carefully around the legs as near to the top as possible and avoiding fouling on adjacent pipes or structure.  A separate tug should be connected to each main undercarriage assembly.  Steering should be carried out by means of a steering arm attached to the nose wheel rather than by differential movement of the tugs.

The most common means of towing a large aircraft nowadays is by means of a tow bar-less aircraft handling tractor.These tractors tend to be front wheel driven and therefore when towing an aircraft, are acting to ‘pull’ the tractor/aircraft combination.The towbarless tractor consists of a low level tractor with a rear mounted cradle, comprising of a ‘scoop and gate’ assembly.

Owing to the tractor’s low height it can easily move-in under the aircraft’s fuselage to couple-up with the aircraft’s nose wheel. During operation the nose wheel is raised by about 20cms by the tractor and after the towing is completed the nose wheel is lowered and the nose wheel is released from the cradle.