The good folks at Goulds Pumps Residential Water Systems share with us this briefing on tail pipe:

Pipe below the jet, or “tail pipe” as it commonly is known, is used when you have a weak deep well. Under normal conditions, the jet assembly with the foot valve attached is lowered into the well. You receive your rated capacity at the level you locate the jet assembly. On a weak well, as the water level lowers to the level of the foot valve (attached at the bottom of the jet assembly), air enters the system. By adding 34 feet of tail pipe below the jet assembly, with the foot valve attached to the bottom of the 34-foot length of pipe, it will not be possible to pull the well down and allow air to enter the system. The drawing to the left indicates the approximate percentage of rated capacity you will receive with a tail pipe.

Using a tail pipe, the pump delivery remains at 100 percent at sea level of the rated capacity down to the jet assembly level. If the water level falls below that, flow decreases in proportion to drawdown, as shown in the illustration. When pump delivery equals well inflow, the water level remains constant until the pump shuts off. This rule also can be used when determining suction pipe length on shallow well systems.
ND