With all the modern security procedures -- belts off, shoes off, laptops out, magnetometers on, etc. -- it seems unlikely the passenger is really armed with explosives. But you can never discount the possibility entirely.
That's the question that was faced by the pilots of EgyptAir Flight MS181 Tuesday morning. The pilot said said a passenger claimed to have an explosive belt, forcing the plane to land in Cyprus.
While it's difficult to imagine a passenger managing to smuggle explosives aboard, in these times, no chances can be taken when lives are at stake.
Security expert Anthony May said it might be possible to smuggle bomb components aboard a plane separately and then assemble them.
At any rate, the pilots of Flight MS181 headed for Larnaca, on Cyprus, and landed safely. The passengers were eventually released. And the alleged hijacker, who officials described as disturbed, was taken into custody.
Here are some questions surrounding the incident:
Q. How could someone board a plane wearing an explosive belt?
A. Well, in this case, no one did. The hijacker, identified by authorities as Seif El Din Mustafa, was "unstable" and it appeared at first that he might have had explosives, Homer Mavrommatis, director of the Cypriot foreign ministry crisis center, told CNN.
But after he gave himself up, authorities determined that his "suicide belt" -- which looked like mobile phone cases -- was fake, according to Alexandros Zenon, the permanent secretary of Cyprus' Foreign Ministry.
At first, authorities had to take the claim seriously out of an abundance of caution. And because it didn't happen this time does not mean it could never happen.
One possible way to do it would be to have inside help, such as an airport security worker.
Q, What is the biggest security vulnerability faced by airlines today?
It appears to be determining how to adequately screen thousands of airport workers.
Many international officials believe a worker at Egypt's Sharm El-Sheikh airport placed a bomb in the hold of Metrojet Flight 9268, causing the plane to blow up over the Sinai desert on October 31, 2015, killing all 224 people aboard.
And the challenge is faced by many countries, not only Egypt.
In November, it emerged that U.S. homeland security officials were trying to address aviation vulnerabilities stemming from more than 900,000 people -- nearly a million -- whose jobs allowed them largely unfettered access behind the scenes at the nation's airports.
But Egypt appears to be a special source of international concern in this regard.
Q: How do you keep an unstable passenger from boarding a plane?
This appears to be an extraordinarily difficult challenge.
Airlines work hard to monitor the emotional health of their pilots. Yet almost exactly one year ago, on March 24, 2015, a mentally ill pilot committed suicide by flying an Airbus A320-211 into the French Alps, committing suicide and killing 149 other people in the process.
The pilot, Andreas Lubitz, had apparently concealed from Germanwings, his employer, his depression and suicidal tendencies.
If airlines cannot ensure with 100% certainty that their pilots are emotionally stable, they certainly cannot ensure that their passengers are.
The best they can do is to keep improving measures to make sure the passengers are unarmed.