by DAVID AXE

For two months, the U.S. Air Force has been without its newest jet fighter. But the situation could be far worse — and likely will be in coming years as the Air Force trades the robustness of a diverse fighter fleet for the supposed cost savings of a more homogenous one.

On May 3, 2011, the Air Force announced it was temporarily banning the Lockheed Martin F-22 from routine flying while the flying branch investigated a series of incidents possible related to the fighter’s oxygen-generation system. The ban was later extended indefinitely, and has forced Lockheed to suspend deliveries of new Raptors.

Above 25,000 feet, the oxygen content of air is insufficient for most people to function normally. In the past, Air Force pilots carried bottled oxygen for high-altitude flights. Today, many military aircraft carry special systems for filtering excess nitrogen from thin air, making it breathable.

But on 14 occasions since 2008, Raptor pilots have reported memory loss or other symptoms of oxygen deprivation, and in November an F-22 crashed in Alaska, killing its pilot. The Air Force grew suspicious of the Raptor’s On-Board Oxygen Generation System, made by Honeywell, and in January limited F-22 flights to 25,000 feet or lower.

The height restriction was a prelude to the May grounding. The Air Force is investigating the incidents but has not said for sure whether the OBOGS is to blame.

The roughly 160 F-22s currently in service represent the main air-to-air fighter for U.S. Air Combat Command and Pacific Command. A force of roughly 250 Boeing F-15C fighters dating back to the early 1980s complements the F-22s. Lockheed F-16s, of which the Air Force possesses 1,000, can also conduct aerial patrols.

Today Raptors at Edwards Air Force Base in California are allowed to fly (presumably at low altitude) for certain unspecified tests, and the entire F-22 fleet remains available for so-called “national-security” taskings — in other words, full-scale war. But after two months with no routine flying, the Air Force’s several hundred front-line Raptor pilots are no longer qualified for combat duty. It would take months to restore the force to full capability.

The Raptor grounding has cut the Air Force’s dedicated interceptor fleet in half, but the diversity and flexibility of today’s air fleet means the flying branch can adjust … and continue fulfilling its responsibilities. But future groundings could be far worse, as the Air Force moves to a fighter force composed of just two types: the F-22 and the smaller, newer F-35.

Cost-efficiency is the major rationale for this “commonality,” although both the F-22 and F-35 have proved more expensive to operate than originally expected.

Around 2030, those two planes should account for the vast majority of the projected 2,000-strong fighter fleet. In those circumstances, an F-22 grounding would be bad, but an F-35 grounding would be a nightmare. In the future, sidelining the F-35 could reduce the Air Force to a token fleet of around 180 F-22s and a handful of surviving (50-year-old) F-15s.

If history is a guide, the F-35 fleet will be grounded at some time over its anticipated 50-year service. Besides the ongoing F-22 flight ban, as recently as 2007 the Air Force briefly prohibited F-15 flights as it investigated structural flaws that caused one of the Boeing fighters to break apart during flight.

Again, the F-15 ban’s effects were mitigated by boosting F-22 and F-16 operations, proof that a mixed fighter fleet is more robust than one dominated by one or two types. For that reason, today’s grounding is an inconvenience for an Air Force, but tomorrow’s could be catastrophic.