– The A-29 attack plane was a white speck in the bright skies over eastern Afghanistan as it launched a dummy bomb that exploded just yards from the target, a wrecked truck. “Spot on!” said a U.S. adviser watching the exercise.

The plane’s Afghan pilot had been guided by an Afghan coordinator on the ground — but only after previous bombing runs had struck well wide of the truck.

Eleven years after the United States began building an air force for Afghanistan at a cost now nearing $8 billion, it remains a frustrating work in progress, with no end in sight. Some aviation experts say the Afghans will rely on U.S. maintenance and other support for years.

Such dependence could complicate President Donald Trump’s moves to extricate the United States from the 17-year-old war against Taliban insurgents — a war in which they lately appear to be gaining ground.

“It would be a home run if we got to 60 to 65 percent” self-sufficiency for the Afghan air force, said John Michel, a retired U.S. Air Force brigadier general, who commanded the air training mission in 2013 and 2014. “You have to have a realistic view of how hard this is.”

For years beginning with the Obama administration, part of the U.S. exit strategy has been to build and train the Afghan military — including the air force — to fight the insurgents on its own.

That strategy appeared to be undermined in December when Trump was said to have ordered preparations for half the 14,000 U.S. troops in Afghanistan to come home.

At the same time, U.S. military officials have been warning that the Afghans remain dangerously unprepared.

“If we left precipitously right now, I do not believe they would be able to successfully defend their country,” Kenneth McKenzie Jr., the Marine lieutenant general nominated to lead the U.S. Central Command, told Congress last month.

Today, U.S.-led coalition aircraft carry out roughly five times as many airstrike missions as the Afghan air force — more than 6,500 last year alone. When insurgents overrun outposts or districts, it typically takes U.S. warplanes and U.S.-trained commando units to drive them back.

By any measure, the Afghan air force is far more capable than the deteriorating, 20-aircraft force of 2007. About 265 U.S.-trained Afghan pilots now fly 118 aircraft supplied by the United States, American trainers said. The fleet is projected to double by 2023.

Brig. Gen. Joel Carey, who commands a NATO-led training mission for Afghanistan, said he was focused on “taking bite-sized chunks and consolidating gains.”

Lt. Gen. Mohammad Shoaib, Afghan air force commander, said the force was not yet big enough to cover the entire country.

Discussing a recent mission failure, he cited the time needed to plan missions and identify targets, as well as delays caused by bad weather, difficult terrain, communications problems and concerns about civilian casualties.

“Afghanistan is a war zone,” Shoaib said. “It’s a challenge.”