Boeing came under renewed pressure Tuesday as the head of Alaska Airlines said inspections carried out after a dangerous incident had found many loose bolts on its 737 MAX 9 aircraft.

The Federal Aviation Administration grounded 171 Boeing 737 MAX 9 aircraft following the incident on 5th January, when a panel blew off the Alaska Airlines plane mid-flight, leaving a hole in the fuselage and forcing an emergency landing.

The company’s chief executive Ben Minicucci told NBC News Tuesday that an in-house inspection of its Boeing 737 Max 9 fleet carried out after the incident had found that “many” of these aircraft had loose bolts.

“I’m more than frustrated and disappointed. I am angry,” he said.

“This happened to Alaska Airlines. It happened to our guests and happened to our people,” he added, calling on Boeing to “improve their quality programs in-house.”

United questions orders

Earlier Tuesday, United Airlines cast doubt Tuesday on future orders and deliveries of Boeing 737s in the wake of the near-disaster, in which no people died or were seriously injured.

In a conference call Tuesday, its chief financial officer Michael Leskinen said 31 out of the 107 aircraft United expects to take delivery of this year are Boeing 737 Max 9 aircraft.

“It is unrealistic at this time to believe all of those aircraft will deliver as currently planned,” he said.