Many years ago, before all the latest stuff, but not before airline terrorists, I worked for Decca-Racal as a marine surveyor.
I travelled with an engineer and we had to take a flight from Norwich to Aberdeen with some sensitive equipment which had to go in the cabin.
The equipment was a sonic "pinger", sometimes called a "fish". It is a device towed behind a survey ship which send acoustic signals down into the seabed so that the echos can be picked up and analyzed in order to detect possible oil/gas bearing strata.
It looked exactly like a guided missile. About 5 feet long, 6 inch diameter, yellow, with fins at one end and a smooth stainless steel cone at the front. The box of tricks with it was an olive-green metal thing about the size of an orange box with a front panel of gauges, switches and gizmos.
The engineer walked down the aisle carrying the "fish" and sat with it in his lap.
I had a seat a little distance away. As we reached cruising altitude, the engineer told me to equalize the pressure on the box to avoid damage to sensitive electronic components. This entailed opening a small valve on the front panel.
The look on the faces of the other pax was already something to behold, but when I gingerly leaned forward and clicked the valve open, an action whch was immediately followed by a "hiss", nearly every passenger dived for cover in his or her seat!
These days it wouldn't be at all funny, indeed it wouldn't be allowed, but at that time, it was quite hilarious.
