Reserves which are too bulky for the reserve compartment of the harness can be relatively difficult–or even impossible–to deploy, especially under high g circumstances. Therefore it’s especially important to ensure that the packed volume of the reserve is not too big for the reserve compartment of the harness. For example, it’s often been found that bulky reserves of an older generation can be relatively difficult to release from compact, modern harnesses.

For some harness models the size (volume) of the harness reserve compartment varies by harness size, which means that each size of harness is compatible with reserves of different packed volumes. For example, for the Advance IMPRESS 4 harness Advance gives the volume of the reserve compartment for the S size as 3–5.5 (min–max) litres, whilst for the M size it’s 3–6 litres, and for the L size it’s 3–6.5 litres.

Tip: For a broad approximation for reserve volume in litres a factor of 2.7 can be applied to the reserve weight in kgs. For example, if the reserve weighs 2 kg then, as a rough guide, the reserve volume is approximately 2 x 2.7 = 5.4 litres (or 5400 cm³). Note that depending on the packing method and skill a rough volume arrived at by this formula, and apparently within approved limits for a harness, still may not release without problem.

If a reserve volume lies within the top third of the certified volume, extra special care must be taken that the reserve is packed to agree with the length of the inner container major axis. In every case proper compatibility checks carried out by the pilot in realistic conditions are the only way to prove that a particular reserve, as it has been packed, will reliably deploy from the harness. When newly packed, the volume of the reserve can be enlarged by 30%. Compatibility checks are strongly recommended after each repack.