Sink Rate
The sink rate is the vertical speed at which a pilot descends under an open reserve parachute. Measured in metres per second (m/s) at the maximum certified pilot weight. Typical values: 4.9-5.5 m/s. EN/LTF limits the sink rate to a maximum of 6.8 m/s.
Also known as: descent rate · sink speed · vertical speed reserve · impact speed
What the sink rate means in practice
The sink rate is not just a number in the datasheet — it is the impact impulse in a deployment. Concrete values:
| Sink rate | Comparable to jumping from … | Impact energy |
|---|---|---|
| 4.5 m/s | ≈ 1.0 m height | minimal |
| 5.0 m/s | ≈ 1.3 m height | noticeable |
| 5.5 m/s | ≈ 1.5 m height | distinct |
| 6.5 m/s | ≈ 2.2 m height | hard |
| 6.8 m/s | ≈ 2.4 m height | EN limit |
EN/LTF prescribes a maximum of 6.8 m/s for certification. Reserves close to this limit are formally safe, but on rocky or uneven terrain the injury risk remains real.
What influences the sink rate
Three main factors determine the sink rate you actually land with:
1. Construction
- classic round (PP) — typically 5.3-5.7 m/s
- square (cross cap) — typically 4.7-5.1 m/s
- steerable (Rogallo) — variable, well below 5 m/s with controlled steering
2. Repack condition
A neglected reserve repack changes the sink rate for real — adhesions delay the opening, the reserve does not open fully, the effective sink rate can be 20-50 % above the datasheet.
3. Pilot load
The closer you fly to the maximum certified pilot weight, the higher your sink rate. A PP 18 at 90 kg pilot load is about 0.3 m/s slower than at 120 kg. The datasheet sink rate is always the worst case at maximum load.
Sink rate vs. impact energy — the square law
Impact energy scales with the square of the speed. That means:
- 5.5 m/s → 100 % impact energy (reference)
- 5.0 m/s → 83 % (-17 %)
- 4.5 m/s → 67 % (-33 %)
A sink-rate reduction of just 0.5 m/s measurably reduces the probability of injury.
Why square reserves have a lower sink rate
A square reserve (Mayday Square) does not fold into a pure hemisphere but into a cross-cap structure. This has:
- a larger effective area at the same packing volume
- less pendulum movement (energy-hungry oscillations are eliminated)
- more stable pressure distribution in the fabric
This is measurable: a Mayday Square 100 at 90 kg pilot load reaches about 4.9 m/s sink rate, a Mayday PP 18 at the same load about 5.5 m/s. The 0.6 m/s difference corresponds to roughly 20 % less impact energy.
→ Detailed comparison: Mayday Square vs Mayday PP
When is a low sink rate critical?
- high mountains with rocky terrain
- hike & fly over rugged ground
- heavy pilot weight near the upper reserve limit
- tandem deployment at high full load
- low deployment altitude — at 30-50 m remaining height, every 0.5 m/s counts
On flat meadow terrain with a normal solo setup, even a 5.5 m/s PP can be landed safely — if you master the roll-away movement on impact.
Where you will encounter it
- When comparing datasheets of different reserves
- When choosing a size — smaller sizes often show higher sink rates at the upper pilot-weight limit
- When choosing a construction — square vs. PP, see above