Altitude tolerance visualizer

Many factors determine how far off a skydiver's estimate of altitude can be and still be considered acceptable, for example: the planned altitudes, where a jumper is within the jump, how fast they are falling, the availability of opportunities to check altitude, whether they are using an AAD, who else is in the air, and what they and the people around them consider acceptable. The chart draws one skydive from exit to landing: the solid line is the jumper’s actual altitude, and the shaded tolerance envelope around it shows the tolerance profile T(A)—the range of estimates that would still allow every future action to be completed safely and acceptably. Above the line the envelope is narrow, because believing yourself higher than you are means acting late; below it the envelope is wider, because acting early costs less—mostly working time, and other people’s plans for the jump—or can even work to a jumper's benefit. The envelope tightens into each threshold—breakoff, deployment, decision altitude, the pattern turns—and changes once each has passed, usually opening up again. The envelope is drawn for one hypothetical jump, a four-way belly jump from 13,000 ft that goes as planned, and its values are those reasoned through in FDS’s theory of skydiving altitude awareness and indication. On a real jump the envelope changes in real time, as things happen: Show a bag lock draws a jump that does not go as planned. You can also show a jumper: set how often they check their altimeter and when they become absorbed (task fixation), and watch the estimate drift between checks and, if it drifts far enough, escape the envelope, indicating a lapse in altitude awareness.

Tolerance at each threshold—on the high side — how much higher than actual the estimate may be and the action still complete in time

How the envelope is conceived. The values are one experienced jumper’s judgment, each made on the side of caution; the monograph’s Appendix B explains the reasoning in detail.

As each threshold is reached the high side is the altitude to spare: ε_max = b − L − Δ, where b is the planned altitude, L the lowest altitude the action may finish by, and Δ the altitude the action uses up (its own cost, plus the descent rate times the time to react and act), so faster disciplines and larger groups tighten it. On this jump: about 500 ft at breakoff and deployment, about 350 at decision altitude; in the landing pattern, where the values follow common practice, 200, 150 and 100 ft.
Between thresholds the envelope runs in straight lines. Right after each threshold it jumps to a new value, because the jumper's reference moves to the next threshold. It opens up only where there is opportunity to check before the next action: widest at the top (2,000 ft just after exit), not at all through the track, when the jumper should be tracking with awareness, a little between the pattern turns, most on the long stretches under canopy.
Decision altitude is an emergency threshold. Its tolerance is computed for the worst malfunction still possible once the canopy is open: a fast spin at about 75 mph, after a stall, a tear or a collision with another canopy, with about 6 seconds to cut away (the low end of a published average) before the 1,000 ft hard deck. Until it has passed it alone sets the high side; after it, with no emergency, the pattern’s tighter limits take over, so the envelope tightens slightly there instead of opening up.
A bag lock is among the highest-speed malfunctions, but can only happen at deployment, so on the planned jump it is ruled out once the canopy opens. Show a bag lock draws a jump where such a malfunction happens and the jumper keeps falling at about 120 mph. It feels like a normal opening at first, so it takes about 4 seconds to fully recognize; once recognized a bag lock is to be cut away at once. The emergency procedures take about 6 seconds, but the release of the main comes early in the process, about 2 seconds in, and must happen by the hard deck, so the procedures must begin by about 1,350 ft, which leaves about 1,450 ft at recognition; an error in the estimate on the dangerous side costs altitude just as hesitation does. But deployment is not finished until a canopy is open, here the reserve, so its own 500 ft still holds, and it is the tighter of the two. Once the bag lock is recognized, the low side opens up to the hard deck: a jumper who believes they are below the hard deck could deploy the reserve without cutting away (SIM 4-1: too low to safely cut away). The RSL opens the reserve at about 1,900 ft, high enough for a canopy control check and to join the pattern in an orderly fashion, so the jumper flies a normal pattern under it with the planned envelope, but with no decision altitude: there is nothing left to cut away.
The low side—believing oneself lower, acting early—is drawn at two and a half times the high side in freefall, one and a half times under canopy above decision altitude, and one and a quarter times below it, where an early turn among the other canopies is partly a matter of etiquette and a late one, nearer the ground, of safety.

The jumper. With a jumper shown, an estimation band traces what they believe: the spread of answers you would get if you could ask them their altitude repeatedly. Its middle is their most likely answer, its width their uncertainty, and its opacity how salient altitude is to them, fading as it slips from mind. While a jumper is shown, the envelope turns gray and the band takes on its colors: green near the actual altitude, orange toward the dangerous limit, red beyond it. Each check corrects the estimate, snapping the band back onto the line. Between checks this jumper’s estimate drifts toward the high side, as it tends to when a jumper is absorbed and time seems to fly (a frightened jumper’s can drift the other way). A lapse is the band leaving the envelope on the high side; the readout below the chart says where the late check comes, and how far below the planned breakoff.

Freefall and canopy. Freefall approaches a belly-to-earth terminal velocity of about 120 mph, and for simplicity the jumper tracks at about the same speed after breakoff (in reality fall rate in a good track is slower than belly); the canopy opens over about 6 seconds before descending at about 15 ft/s—constant for simplicity. The time axis gives freefall extra width, and the landing pattern, or the bag lock, is enlarged in an inset. Every number here is a model, not a measurement.

This is not to be taken as a recommendation for a jump. Nothing here is a substitute for proper training by certified instructors. See the United States Parachute Association’s Skydiver’s Information Manual, whose minimum breakoff, container opening and decision altitudes this page uses.