The lost chord

Not long ago, it is said, the passengers of an airliner heard this message over the intercom: ‘Good afternoon ladies and gentlemen. This is your pilot speaking. We are flying at an altitude of 35,000 feet and at a speed of 610 miles per hour. I have two pieces of news to report, one good and one bad. The good news is that we are making excellent time. The bad news is that we are lost.’