Three hardovers have occurred since mid-2000, spurring on work to develop a long-term solution for Tomcat

The US Navy is urgently developing a long-term fix for a rudder hardover problem affecting the Northrop Grumman F-14 Tomcat, and has developed emergency procedures to allow limited operations in the short term.

Three rudder hardovers have occurred since mid-2000, one of which contributed to the loss of an aircraft. The most recent event, which occurred in January this year during air combat manoeuvring exercises, resulted in an emergency landing and damage to the aircraft which left the runway at Oceania Naval Air Station.

Following the incidents, the USN barred the F-14 from carrier-based operations too far from suitable land-based diversion airfields. However, a carrier-based F-14 squadron was returning from deployment at the time and, in view of the operational needs to maintain flight status and to avoid the costs of craning off an entire squadron, a set of emergency procedures were quickly developed.

The time pressures and complexity of safely flight testing a rudder hardover meant that the Naval Air Warfare Center Aircraft Division team tasked with the job had to revert to using the F-14 manned flight simulator at Patuxent River, Maryland, to find a solution.

Investigations, meanwhile, traced the problem to the mechanical failure of a stop plate attached to the rudder actuator. The plate limits the servo valve motion as well as sealing the hydraulic system, and its failure led to the hardover in the affected rudder and rapid loss of hydraulic pressure. The long-term mechanical solution to the problem is therefore focused on correcting the stop plate issue.

Using the experience from the pilot involved in the Oceania incident, the team developed an accurate simulation of the failure. Simulator tests were run in February and March and covered reactions to 'up and away' rudder hardover failures including pilot reaction and the steps needed to recover the aircraft. The team also examined the transition from recovery to the controllability check and looked at various configurations at low altitude with flaps up and down, as well as shore- and shipboard approaches and landings.

Pratt & Whitney TF30 and General Electric F110 engine models were tested, with the older, lower power P&W engine considered the 'worse case' scenario.

Results in up and away conditions show the aircraft can always be recovered regardless of airspeed; however failure at high angle of attack and high g conditions results in the loss of up to 8,000ft (2,440m) of altitude. Tests in the powered approach condition show a much more dangerous situation with slim chances of recovery. Worse still, they showed that "staying within the ejection envelope within the first few seconds was often impossible."

New Naval Air Training and Operating Procedures Standard-isation recommendations issued to the fleet now include the suggested use of manoeuvre flap approaches for the best landing configuration, the use of differential thrust in turns, full flap approaches only with extreme caution, and the immediate jettisoning of all drop tanks and ordnance, but only if done symmetrically.

In the event of a problem, pilots are advised either to divert to shore, or if landing on the carrier, not to use barricade arrestment.

Source: Flight International