Aircraft tie down, or picketing action, is taken to minimise the possibility of movement of a parked, non-hangared aircraft due to high winds or propeller wash / jet-efflux from taxiing aircraft. Tie down is normally applicable only to smaller aircraft unless extreme conditions are expected.
Every year many light aircraft are unnecessarily damaged due to the effects of strong winds or exposure to propeller/jet efflux from adjacent aircraft. This can be a consequence of inattention to weather forecasts, their inaccuracy or to failure to recognise their significance. It may also be a consequence of inappropriate tie down procedures and/or failure to position the aircraft into wind before tie down (see below). The judgment about whether tie down is a necessary precaution must take into account the actual weight of the aircraft involved relative to the expected maximum wind speed.
When tie down is to take place, an aircraft should be positioned into the forecast mean wind direction and if significant change in this is expected during the period of tie down then arrangements to reposition the aircraft at some point may be necessary. In the case of small tail wheel aircraft, it is often suggested that when positioned nose into wind, the elevators should be secured in the up position. The opinion has also sometimes been expressed that they should actually be tied down tail into wind but in this case, the installation of external Flight Control Locks with the elevator secured in the down position and the use of a tie down point at the tail would be essential.
To be effective, the tie down points or weights must be appropriate for the actual weight of the aircraft involved. If parking with fixed tie down points or pre-positional heavy weights is not available then parking on grass and using stakes or pickets carried on board or otherwise obtained is the only option. Pickets depend for their effectiveness on both their design and their use. Picket design includes both crossover tubes which are used to enable a pair of straight stakes to be put into the ground at a 70-80 degree angle and spiral rods. The latter can be difficult to get into stony ground and may be more likely to pull out of soft ground.
In the case of both tie down to fixed points or heavy objects, the nature of the rope used and the knots tied using it may be of crucial importance. Nylon or similar rope is usually preferred and for light aircraft, rope rated as capable of sustaining a 3000 lb (1400kg) load is usually considered suitable for single engine light aircraft purposes, 4000 lb (1800kg) for light twins. Appropriately rated and attached shackles must also be used - weak alternatives to rated shackles are often the weakest point in a tie down or picketing regime. Poor knots are the other potential weakest point in this situation - use of bowline or reef knots is recommended in most guidance material.
If a fully satisfactory tie down solution is not available then consider taking advantage of any available shelter and seek local advice in this respect if possible. If a relatively sheltered place cannot be found, it may be possible to park a large truck or tractor in front of the aircraft to help break up the airflow at the aircraft, although this action can sometimes introduce an additional risk by exposing the aircraft almost simultaneously to a range of wind velocities.
Risks associated with Tie Down
- Structural damage to an aircraft if the tie down is improperly implemented or the wind exposure exceeds its restraining ability.
- Impact damage if objects in the vicinity, including other aircraft, or pieces of debris detached from adjacent buildings are blown into the aircraft.
- Failure to detect damage caused, especially to the flight controls, prior to flight with the consequent risk of loss of control.
- Failure to disconnect the aircraft from tie down blocks before attempting to taxi or taking off.
Alternative Risk Management
- Hangar the aircraft.
- Fly it to another aerodrome where the forecast wind speed is less.
Accidents and Incidents
On 26 May 2013, about 20 minutes after arrival at Singapore for a turn round expected to last about an hour and with crew members on board, a Boeing 737-900 was suddenly rotated approximately 30 degrees about its main gear by a relatively modest wind gust and damaged by consequent impacts. The Investigation concluded that the movement had been due to the failure to follow manufacturer's guidance on both adequate chocking of the aircraft wheels and the order of hold loading. It was found that the Operator had not ensured that its ground handling agent at Singapore was properly instructed.
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.