The airline safety instructions voiced over by air hostesses at the start of each flight are similar the world over, delivered in a typical monotonous tone and paid little attention to by frequent air travelers. My guess is most people don’t know the logic behind some of the instructions (at least, I didn’t) and how do they help. I looked up on the internet and found some answers. 

1. Why do you have to keep your cell phone switched off during the course of the entire flight? 
A. This one’s the most obvious instruction and the logic is understood by most people. Cell phones receive and transmit wireless signals and these signals can interfere with the aircraft communication systems.

2. Why do airline seats have to be in an upright position during takeoff/landing?
A. Air & Space magazine answers this one well. 

There are two main reasons why flight attendants pester people to keep those seats up—to keep injuries to a minimum during a crash and to clear the maximum amount of space for a quick exit. When the seat is up, it is locked. When the seat is back, it’s not locked. In the event of an emergency, an unlocked seat has more force during impact, and the thrusting forward of that seat can cause passenger injury.

3. Why do you have to fold up your tray tables during takeoff/landing? 
A. Same logic as above.  

4. Why window shields have to be slid up during takeoff/landing? 
A. This rule is in place to provide visibility into the aircraft during emergencies. 

5. Why do you have to switch off all electronic equipment during takeoff/landing? 
A. This one’s trickier. The rule to switch off cell phones is understandable, but why should other electronic equipments like iPods need to be switched off. After all, they do not transmit/receive any signals. Right? Wrong. An iPod and most electronic devices emit a small amount of RF signals (in case of a malfunction, the strength of the RF signals may go up). While a single iPod may not emit signals strong enough to cause interference with aircraft communication systems, a bunch of them running together might (the chances are remote, but the airline companies don’t want to take a risk and face litigation).