In the earlier days of radio control, failsafe systems were not used. If the RF link was broken, the aeroplane would typically fly away.

This is unsafe as the aeroplane could come down in a crowded urban area causing property damage or injury, or fly upwards and collide with full size aircraft.

With the advent of modern digital RC designs, it has become easy to incorporate failsafe systems in modern receivers and this is now a legal requirement, i.e. if the radio system used supports the failsafe feature, then it must be fitted and correctly set up before flying. Older RC systems which do not support the failsafe feature are still legal at the time of writing however.

The purpose of failsafes is detailed in the BMFA's (British Model Flying Association's), members handbook, where reference is made to the CAA's (Civil Aviation Authority's), requirement that failsafes are required to prevent fly aways.

This means that if failsafes are commanded by the receiver because of loss of RF signal, the aeroplane must come down in the vicinity of the flying field. How the aeroplane comes down however depends on:

1. The Failsafe strategy the model pilot chooses.
2. The design of the aeroplane.

This means there is a requirement on the part of the model pilot not to leave the failsafe device set at defaults but to make the best choice of failsafe strategy according to needs, then get it working.

Failsafe strategies:

1. Throttle only. Throttle is reduced to idle. Aeroplane descends. CAA's requirements are met.
2. Hold last position. The last position of control surfaces and throttle, prior to the failsafe event, is held on all servos. E.g. if you were commanding full throttle and right aileron, when the failsafe was commanded, the aeroplane would hit the ground at full throttle, probably vertically, while rolling, or if you were in straight and level climbing flight, the aeroplane could fly away and collide with full size aircraft. CAA's requirement's are not met.
3. Programmable. Allows the user to program all servos so that when a failsafe event is triggered, flying surfaces and throttle go to pre-programmed positions. CAA's requirements are met, providing the failsafe is correctly programmed by the user.

Model Pilots need to understand the full implications of these strategies, so they are detailed next.

Throttle Only: 

Meets the CAA's requirements, i.e. aircraft comes down in the vicinity of the flying field, typically at idle, i.e. low power. There is more to it than that however. Typically while no power is given to flying surface servos from the moment of the failsafe, they retain the position that they were in at the moment of the failsafe. Further because of the servo gear train's resistance to backfeeding caused by it's gear ratio, they stay there, effectively as if commanded. This means if you are commanding full throttle and full aileron at the instant of the failsafe event, that while the engine will cut to idle, the ailerons will stay at full deflection with the aeroplane probably coming down vertically while rolling, picking up speed as it does so, the aeroplane will probably be wrecked or worse. It still meets the CAA's requirements.

Hold Last Position:

This is not suitable for radio controlled model aircraft use, as throttle is not cut, and so could lead to dangerous high speed powered descents or flyaways.

As Hold Last Position can be the default setting in programmable failsafes, the model pilot must ensure this default is not set. It is also worth pointing out that the last position is held by servo command, i.e. servo's actively maintain the last position.

Programmable:

If used, this is typically set to throttle cut to idle and flying surfaces set to generate a gentle turn, still meeting the CAA's requirement.

Please note whether this last idea of the gentle turn can be successfully implemented depends on the aeroplane's design, i.e. an aeroplane with good stability in the roll and pitch axes, could implement this well. Stability in roll means dihedral, stability in pitch means decent tailplane moment arm and surface area. (See Neutral Point and Static Margin)

Performance aeroplanes e.g. pattern ships, typically do not have such stability and so typically would come down out of a gentle glide pattern and so could be damaged on impact.

NOTE: Not all radio systems offer programmable failsafes. This means that if a model pilot desires this feature, he must ensure he chooses a radio system that offers it.

Safety at the flying field.

While the CAA's needs are met thanks to the correct implementation of failsafes, i.e. people in urban areas are protected from model aeroplanes descending out of control and people in full size aircraft are protected from uncontrolled flyways, people on the ground at the flying field are not. i.e. the risk is there at the flying field. The risks at the flying field are normally handled by good field safety protocols. e.g. a model pilot will typically be required to shout "RF Lost" repeatedly so that others present can get eyes on the aeroplane with no control as it descends.

For this important safety reason, model pilots should do everything possible to ensure their RF link is robust, if not bullet proof, thereby minimising the risk that  failsafe events present at the flying field.

Related Article Links:

2.4 gHz. RF link problems 1. New equipment failure.

2.4 gHz. RF link problems 2. Tx/Rx RF link principles.

2.4 gHz. RF link problems 3. Advanced Range Test Protocol.

2.4 gHz. RF link Problems 4. Causes and Fixes

2.4 gHz. RF Link Problems 5. Failsafe Triggered

2.4 gHz. RF link Problems. 6. Failsafe Strategies.

2.4 gHz. RF Link Problems 7. Faultfinding.

2.4 gHz. RF Link problems 8. Failsafe Recovery Time.

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