ISLAMABAD — Over a dozen Pakistan Air Force (PAF) planes have crashed in roughly the past 18 months, raising concerns about the health of an aging fleet that officials are struggling to upgrade because of a lack of funds. A significant number of the air force's combat aircraft are nearly half a century old and have been called on in recent years to help the army fight a domestic Taliban insurgency that has killed thousands of people. This has added to the strain on a force that has historically focused on countering the threat from Pakistan's neighbor and archenemy, India. Pakistan has turned to the US and China for help in modernizing its air force, but economic woes have strained the government's budget, even for the country's powerful military. Tension with the US over a host of issues, including the covert raid that killed Osama Bin Laden last year, also hasn't helped. Shahzad Chaudhry, a former senior PAF officer, said the number of planes that have crashed “would be worrying for any air force.” “If I were air force chief, I certainly wouldn't want to see over a dozen airplanes being lost,” said Chaudhry. “Obviously we need to bring those numbers down.” At least 13 planes have crashed since May 2011, many because of technical problems, according to a record maintained by The Associated Press. The PAF did not respond to request for comment on the crashes. The most recent accident occurred on Nov. 22, when a Dassault Mirage fighter jet crashed on a routine night training mission in central Punjab province, killing the pilot. The air force said at the time that it was investigating the cause of the crash, but eyewitnesses said the plane caught fire before it fell out of the sky. Nearly half the planes that have crashed were decades-old Mirage fighters. They make up at least a quarter of the force's fleet of about 400 combat aircraft, according to the website GlobalSecurity.org. Many were built nearly 50 years ago and acquired from foreign militaries that had already retired them. “We bought them at almost throw-away prices, brought them over, overhauled them and continue to operate them,” said former PAF chief Tanvir Mahmood. “This was our compulsion because of the financial constraints that we had.” Chaudhry, the former senior air force officer, said the Mirage was a “very sturdy platform” that shouldn't fail easily. “But when you have the intensity of an operational combat environment, problems tend to be there,” said Chaudhry. The crashes raise questions not only about the age of the aircraft, but also flight maintenance practices, said Sajad Haider, a celebrated former air force pilot who has written a book about the service. Other planes that have gone down include the Chengdu F-7 fighter jet bought from China, the JF-17 Thunder fighter jet jointly developed with China and Pakistan's Mushshak trainer, a propeller plane. The most advanced fighter jet operated by the PAF is the F-16 Fighting Falcon, over 60 of which were purchased in various batches from the US over the past three decades. — AP