By Humeyra Pamuk

ISTANBUL (Reuters) - Turkey's air force in September made a public appeal to hundreds of former pilots to return to its depleted ranks to replace more than 350 airmen purged after July's failed coup. 

The call has largely gone unanswered, according to military officials and former pilots, as the deep divisions exposed by the coup attempt in Turkey's military and other institutions remain unhealed.

Six pilots who left the air force over the past decade and would be eligible to return spoke to Reuters. Only one said he would re-register to help replace dismissed colleagues whom the government blames for being part of a network that planned the failed July 15 coup.

Air force pilots played a major role in the abortive putsch, commandeering jets and helicopters that bombed parliament and threatened the aircraft of President Tayyip Erdogan.

Turkish media have reported that only a tiny proportion of the several hundred pilots who left the force between 2010 and 2015 and would be eligible to return have actually re-registered. The numbers are "below expectations," said a military source who is not authorized to speak publicly.

The depletion in air-force resources comes at a delicate time for Turkey, the second-most militarized member of NATO. Ankara is pursuing military incursions against Islamic State in Syria and battling Kurdish PKK militants in Turkey and northern Iraq.

Yet the former air force pilots contacted by Reuters said they felt betrayed by a military which had failed to protect them from followers within the ranks of Fethullah Gulen, the U.S.-based cleric Turkey blames for the attempted coup and whose supporters have been jailed en masse since.

COMMERCIAL AVIATION

Working in commercial aviation is also more remunerative than air force salaries of less than 10,000 lira ($3,200) a month.

"Flying an F-16 was an honor for me and I have been unfairly deprived of that. I feel betrayed. Why should I come back?" said a former major who only gave his first name, Mehmet, for fear of being seen as disloyal.

Mehmet – who graduated at the top of his pilot training course and has a post-graduate diploma from an international military school – said he was grounded in 2008 by military doctors who said he had a heart condition. He left the air force and built a career as a commercial pilot after other doctors gave him a clean bill of health, he said. Reuters could not independently confirm his medical status.

The military doctors who grounded him are now in jail, accused by the government of being members of the Gulen network.

Government officials deny that the coup and its aftermath have damaged Turkey's military capabilities. They acknowledge staff shortages but say that a more loyal and focused military will ultimately emerge. The defense ministry and military command both declined to comment for this story.

The military source said the air force was considering a reserve system to allow commercial pilots to take temporary contracts, a move that could help lure back experienced fliers because it would allow them to maintain their lucrative commercial contracts at the same time.

The defense ministry also plans to start enrolling students from private universities to the Air Force Academy, widening its pool of candidates by dropping a condition that they must have attended military school.

"In two years time, you will see, our air force will be stronger than before," Abidin Unal, the top air force commander, told Turkey's Vatan newspaper two weeks ago.

INFILTRATORS

The Turkish government blames Gulen and his sympathizers for infiltrating state institutions over decades and ultimately masterminding the July 15 coup attempt.

Erdogan's critics, particularly Turkish secularists, say he and the Islamist-rooted AK party he founded allowed Gulen's followers to rise through the ranks to help control the military. Turkey’s military institutions have long seen themselves as the guardians of the country's secular order and have ousted four governments since 1960 for posing what they saw as an Islamist threat.

The Gulenists' impact was particularly acute in the air force, former military officials say. Several former pilots said military doctors struck off colleagues on what the pilots said were minor medical grounds as they tried to remove non-Gulenists from senior positions.

"I can count dozens of people like this ... It seemed that some people were getting rid of others who didn't necessarily think like them. But we could never be sure, so we didn't speak up enough," the second former air force pilot said.

Air force pilots were prominent in the coup attempt on July 15, commandeering fighter jets and helicopters in part of the bid to seize power. Fighter jets flown by pilots involved in the attempted coup harassed Erdogan's aircraft while he tried to return to Istanbul as the events unfolded.  

More than 350 pilots and 40 technicians have been dismissed, detained, or are being sought.

NERVE CENTER

Among those detained were the former head of the air force and several commanders of air bases including Incirlik, a hub for the U.S.-led coalition against Islamic State. Three squadrons were taken off duty at the Akinci air base near Ankara, nerve center of the failed coup and home to some of the force's top pilots.

"Those who were dismissed mostly consisted of either experienced pilots or junior ones who were in the process of receiving weapons and tactics training," said Arda Mevlutoglu, a defense industry consultant.

"It can safely be said that Turkey's air force had lost a huge portion of experience and know-how because of this coup attempt," he added.

Turkey does not disclose the total number of its combat pilots, but its Defence and Aerospace magazine quoted the head of the air force in March 2015 as saying overall pilot numbers were expected to rise to 1,300 that year.

Turkish media have reported that the ratio of jets to combat pilots has dropped to below the 1:1.5 considered a healthy standard internationally, although there is no NATO rule on what the ratio should be. The air force has 240 F-16 jets and 49 F-4s for combat use, according to its website.

One NATO diplomat said that while there was concern about the purge of the Turkish air force, it had not yet resulted in any change to Turkey's commitments to NATO operations.   

Faced with the shortfall, the force appealed in September for pilots with combat experience to rejoin, a call launched on its website and announced by the defense minister and widely publicized in the Turkish media.

Ersoy Cil, 40, a former F-4 pilot who left the military in 2015 after nearly two decades, is now returning.

"I never would have had the opportunity of a university education if it wasn't for the force," he said.

Cil’s parents would not have been able to afford a private university education but he was able to go through state-funded higher education after graduating from military school.

"This is me trying to pay back my huge debt.”

But the others contacted by Reuters all shared a sense of betrayal for what had happened in the past.

“If there was an emergency situation, I'd go running,” said a pilot who left the force in 2013 after more than a decade patrolling Turkey's borders and fighting the PKK. “But apart from that, the moment has gone, I've set up a new life."

(Additional reporting by Robin Emmott Editing by Nick Tattersall, Giles Elgood and Alessandra Galloni)