Australia’s national airline, Qantas, this week grounded three of its Boeing 737s after cracks were found in a part of the plane known as the “pickle fork”.

The plane – the 737-NG to be precise – is one of the most commonly used models in the world, with over 7,000 in service globally.

So far, 50 aircraft around the world have been found with cracks, including nine in South Korea and three belonging to Southwest Airlines in the US. The Australian aircraft engineers union has called on Qantas to ground all of its 737s.

But Qantas and other affected airlines say that they have completed the relevant checks, and are proceeding with an abundance of caution.

Dr Garth Pearce, a senior lecturer in aerospace engineering at the University of New South Wales, says this should be enough to ensure the safety of all passengers.

What is the pickle fork?

The pickle fork lies between the wing and the fuselage. It strengthens the connection between the wing and the body of the plane.

But, as this article from the ABC points out, the pickle fork is not the only thing that holds the wing in place.

What are the airlines doing?

The issue is related to the number of flight cycles a plane has gone through.

A cycle is one take-off and landing – with most commercial planes going through roughly 1,700 cycles a year.

Last month, a global aviation regulation was issued requiring all Boeing 737s with more than 30,000 lifetime cycles to be urgently checked – within seven days.

737s with between 22,600 and 30,000 also need to be checked, but within seven months. Planes below 22,600 do not need to be checked until they reach that milestone.

This week, Qantas discovered that three of its planes that were below the 30,000 mark had cracks (at 27,000 cycles).

The airline then urgently checked all 737s between 22,600 and 30,000 cycles, completing this within seven days rather than the seven months required. The airline does not have any planes above 30,000.

Globally, Boeing has checked all planes above 30,000, and a third of those above 22,600. Of the 1,000 planes they have checked, faults were found in less than 5% of them.

How many planes could be affected?

This may vary from airline to airline, but we can use Qantas as an example.

Out of a total 75 Boeing 737s, Qantas has none above 30,000. There are 33 above 22,600 and 42 are below 22,600.

Qantas has said it will not check any of the planes below 22,600.

Ryanair has more than 400 737s in service, but has not disclosed whether it has discovered any issues so far. Virgin Australia has inspected all 19 of its 737s over 22,600 cycles, without findings cracks. American Airlines and United Airlines said they are inspecting their entire fleets but have not found any cracks so far.

Is it safe to fly?

The Australian aircraft engineers union has called on Qantas to ground all of its 737s. But the airline has said it is confident that, after checking all those above 22,600, it does not need to take further action.

UNSW’s Pearce told Guardian Australia this was sufficient. He said that a small percentage of planes would be affected, and that the risk of cracks in those with fewer cycles would significantly drop.

“If 5% of all inspected aircraft [over 30,000 cycles] have been found with cracks, that number is certainly going to be lower in the the aircraft that have flown fewer flights,” he said. “It’s going to be a smaller number than previously found.

“This whole process shows that regulation really does work in the aerospace industry. A crack was found on one aircraft and all of a sudden all the airlines around the world are notified, and are searching and are rectifying it.

“These parts, in addition, are designed to be very tolerant of small cracks. [T]he fact that this incident occurred is not a scandal, it shows the industry is working as expected.”

Pearce said he did not think airlines needed to follow Qantas’s lead in accelerating the check of the 22,600 to 30,000 cycle planes from seven months to one week.

“Qantas has done the right thing from a public relations perspective and gone and done it on an accelerated timeframe. But I don’t think that’s a pure safety decision. It is showing that they are being proactive, and showing the public that they really do care.

“They are going over and above. I wouldn’t recommend that airlines do anything other than what the regulator asked them.”

Even though the 737 is very common, he said the checks would not cause significant global disruption to flights, as it would not likely rise above 5% of planes.

Boeing has also said that the checks have “mainly been focused on the oldest aircraft, so the percentage of aircraft with cracks found should reduce as newer aircraft are inspected”.