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Aircraft Tie Down
Aircraft tie down or picketing action is taken to minimise the possibility of aircraft movement due to high winds or propeller wash / jet-efflux from taxiing aircraft when not hangered. 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 of 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 taking off.
Alternative Risk Management
- Hangar the aircraft.
- Fly it to another aerodrome where the forecast wind speed is less.
- B738, vicinity Faro Portugal, 2011 (LOC AW WX) - 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.
- Jet Efflux Hazard
- Prop Wash
- Flight Control Locks
- Flight Control Protection from Damage by Strong Winds
- Secure Your Aircraft, Good Aviation Practice (GAP), CAA NZ, 2004
- FAA AC20-35C, Tiedown sense, December 1983
- Tie me aircraft down, sport - tie down tips to keep your aicraft secure, a Flight Safety magazine article, CASA, March-April 2006