A tow launch concentrates risk in a few seconds near the ground, and its three characteristic failures are a tow that cannot be released, a rope or weak link that breaks at low height, and the stall that follows when a nose-high aircraft loses the pull. The cockpit release must work under load every time and be tested before every launch, the pilot lowers the nose before doing anything else when the rope goes, and the rope and weak link must be tested rather than assumed. If you cannot meet all of those, do not tow.
Part of Aerodynamics and Flight
Launching an unpowered aircraft by vehicle or winch, the rope and release hardware, and how each failure ends.
Why The Tow Is The Dangerous Part
For several seconds the aircraft is low, slow, pulled from a point ahead and below, and dependent on a rope, a hook and a driver. Everything that can go wrong does so near the ground, with little height to recover. The carrier treats the tow as a step in a progressive sequence (Glider Testing); this article is about doing that step without dying in it.
The Three Failure Modes
- A release that does not release. If the pilot cannot let go, the aircraft is dragged toward the vehicle, or is pulled up into a stall by a rope that will not let it fly free. Friction in a release under tension is far higher than unloaded, so a release that works on the bench can jam under load.
- A rope or weak-link break at low height. The aircraft is nose-high, the pull disappears, speed falls, and the wing is near the stall.
- The nose-high stall after the break. A climbing attitude on tow with the sudden loss of tension leaves the aircraft above the angle it can hold at its remaining speed (Stall Dynamics). The only recovery is to lower the nose at once, which at low height costs height the pilot may not have.
Release: Requirements
- A cockpit release operated by one hand without letting go of the controls, and reachable in any attitude.
- It must release under full load; test with a heavy pull on the rope, such as the vehicle holding a tension with the aircraft chocked.
- A second release at the vehicle end, so the driver can drop the rope.
- No knot in the release path. A knot under load cannot be undone.
- Tested before every launch, by the pilot, with a witness.
Rope And Weak Link
The tow rope sees steady tension and shock loads. Find its real breaking load by testing samples from the same coil to failure (Rope Strength Testing), but the safe-working fractions given there are for shelter lines and people, not for a tow. A weak link, a deliberately weaker section, is fitted so that an overload breaks the link instead of the aircraft’s fitting or the airframe; its strength is set from the aircraft’s own tow-load limit, which you must have established by calculation and which is not given here. Check the rope for damage and wear before every launch (Rope Care); plant-fibre rope degrades unseen.
Method: A Cautious Tow
Step 1: Brief everyone. Pilot, driver, wing-runner and an observer agree signals and what happens at each failure.
Step 2: Check release and rope as above.
Step 3: Keep the tow low and straight. Climb gently, with the speed held where the aircraft is clearly above its stall. Early tows are a few metres up.
Step 4: On any problem, release. The pilot pulls the release and lowers the nose first, then looks. The driver stops pulling.
Step 5: Land straight ahead on the cleared strip. No turns at low height.
Common Mistakes
- Practising the break only on paper.
- Towing in gusts or crosswind.
- A rope that can drape across the vehicle or a person.
- Climbing steeply.
Safety
Release, rope break and stall
These are the recognised ways a tow launch kills. Do not tow with an untested release, an untested rope, a driver who cannot see the aircraft or a field without a clear landing area ahead. A person on the vehicle must be able to drop the rope.
What’s Next
- Learn next: Hill Soaring, flight that follows a tow
- Combine with: Envelope Expansion, the discipline of widening the steps