Feb. 20 -- The Federal Aviation Administration is investigating of series of errors last Tuesday that caused three planes to land too close together in rapid succession here.
In each case, the amount of space between the planes was compromised by slightly more than a tenth of a mile, which is not enough to be seen by the naked eye but is a breach of the separation rules air-traffic controllers are charged with maintaining.
"It's very unusual; very, very unusual to have three incidents at one time," said Kathleen Bergen, FAA spokeswoman. "None of them in and of themselves are serious, but our standards do remain constant involving heavy jets and smaller jets."
The air-traffic control tower reported one incident. When the FAA began investigating, it found three breaches, she said.
The string of events started shortly after noon when FedEx flight 881 approached for a landing on the center runway.
While still in the air, the pilot reported an auto throttle problem to the tower, which required the plane to slow faster than usual.
When that happened, the plane coming in behind it, a regional jet operated by Pinnacle Airlines, broke the 5 miles of separation the FAA mandates between heavy and small jets.
The FedEx pilot aborted the landing and flew around to try again. As it approached the runway for the second time, the separation between it and another FedEx plane at an adjacent runway was compromised.
"The diagonal separation between those planes should have been 1.5 miles. But because flight 881 again reduced its speed, the separation was reduced to 1.38 miles," Bergen said.
As both FedEx planes were preparing to land, another regional jet operated by Pinnacle was approaching for landing.
Its 5-mile separation was reduced to 4.86 miles.
"The normal procedure we do with every error is to decertify the controllers and put them through some form of retraining," Bergen said.
The FAA has strict separation rules in part to keep small planes from flying into the air turbulence created by large planes.
So far this year, the Memphis air-traffic control tower has reported four operational errors.
The year began Oct. 1.
"In all three of the operational errors, we had more than 95 percent of the required separation. But our job is not to have loss of any," said Peter Suflaw, head of the air-traffic control union at the tower.
Last year, the Memphis tower reported 10 errors and was asked by the FAA to come up with a plan for reducing errors.
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Decertification is designed to identify areas in which employees need retraining and is not considered disciplinary.
"If FedEx can't land as many planes here, they will have to move some operations to another hub."
The FAA estimates a more than 15 percent increase in capacity at Memphis as a result of RECAT.
Air traffic is typically light at that hour and no flights were affected.