A probe line search is appropriate when the victim cannot be located using a transceiver or by other readily available means (e.g., RECCO, spot probing, dogs, etc). A probe line requires a large number of rescuers and is a slow process. It is usually considered a recovery, rather than a rescue, technique. Probe lines are typically instigated by professional rescuers (e.g., ski patrols or SAR teams) and are rarely used during companion rescues.
A probe line involves a row of rescuers repeatedly inserting avalanche probes into the snow and then advancing. It is an effective albeit slow way to search an avalanche.
Probe line searching is often called "organized probing," but your probing should always be organized regardless if whether there is one person or 20.
A probe line typically has a leader, probers, and possibly two people who manage a guidon cord ("guide-on"). The probers can be bystanders who are trained on the spot.
The following slideshow illustrates the probe line process (click on the right-arrow to advance through the slides; click and hold an image to prevent it from advancing).
When used in a probe line, the probes should be held vertically relative to gravity (i.e., plumb). When probing after a transceiver search, the probe should be held at 90-degrees relative to the snow surface.
If there is a chance the victim is still alive, you may want to probe to only 1.5 meters (5 feet) so you can cover more terrain in less time. However, probe lines are almost always considered a recovery technique. In that case, rescuers should probe the full length of their probes.
These instructions assume 50 cm (20 inches) between probe holes. This "coarse" spacing has about an 80% chance of finding a person on the first pass. It's a good balance between speed and thoroughness.
If you don't locate the victim using coarse space, you can consider using fine spacing. In fine spacing, rescuers line up elbow-to-shoulder, probe three times (with approximately 25 cm [10 inches] between probe holes), and take small (25 cm [10 inch]) steps forward. The fine spacing takes twice as long with a relatively small increase in detection.
Probers. It is much easier on the probers' backs if they face uphill. This also gives them better situational awareness—not that they could outrun an avalanche. The probers can achieve the desired five-foot spacing by standing palm-to-palm.
Lines People. The optional lines people can hold a guidon (pronounced "guide-on") cord that is typically marked (either with paint, knots, or ribbon) every 50 cm (20 inches). The markings help keep the probers in a line and ensure proper spacing between probe holes, but the spacing will be good even without a cord provided the probers maintain the five-foot spacing.
The most important task performed by the lines people (and this can be done by the probers on the ends of the probe line) is to periodically place flags adjacent to the outside probe hole. Knowing the terrain that has been probed is critical as the incident commander adapts the search plan.
Leader. The leader is ultimately responsible for the entire probe line search. In addition to giving the verbal probing commands, the leader must ensure that proper spacing is maintained between probers and that the probers remain focused. The leader can observe the heights of the inserted probes to gauge the underlying terrain versus a possible probe spike. Additionally, the leader may need to train bystanders so they can participate in the probe line.