Question: I have been on three flights that have made emergency landings that required diversions, assuming the "head down, stay down" position during the landing, and having the runways foamed.  I still fly out of necessity and manage okay except during the descent, which almost always involves lots of shaking and roller coaster drops. It's worse on the smaller planes.  Can you please tell me exactly what's going on during the descent, and if it's as dangerous as it seems? 

— Elizabeth Koeninger, Sarasota, Fla.

Answer: It is not dangerous. Flying is the safest form of transportation ever created by mankind.

Unfortunately, your experiences have been a bit traumatic. It is very, very unusual for a passenger to have been in three landings where evacuation preparations were in effect. That is more than I have been involved in, in over 48 years of flying.

I appreciate your apprehension but please be assured that the descent is not a roller coaster ride. Descending from cruise altitude can take many forms due to the requirements of air traffic control. In some cases, it is necessary to descend quickly to meet crossing restrictions. Pilots practice this frequently and airplanes are designed for it. There is no problem with this.

A smaller airplane may provide more sensation of rapidly descending, but they usually fly nearly the same profiles as larger ones.

Last year we flew nearly 4 billion passengers on nearly 40 million flights without a passenger fatality in a jet. There is no question flying is safe.

More: Ask the Captain: Why aviation was so safe in 2017

Q: At what distance from its destination does an airliner begin its descent, and is the descent executed at a uniform rate or in phases?

— Steve, Ohio

A: Normally an airliner will begin its descent around 100 to 120 miles from the destination (assuming the cruising altitude is above 30,000 feet).

The point of initial descent varies depending on the cruise altitude, the amount of traffic going to your destination and weather conditions. The rate will vary depending on crossing restrictions and the wind.

Q: How long does it take from the start of descent to touchdown? And how many feet per second does the airplane descend?           

— Avi

A: It varies depending on traffic and the needs of air traffic control. Modern airliners have flight management computers that will begin a descent from 30,000 feet, approximately 100 miles from the destination planning an idle descent. The computer calculates a vertical path based on the approach and landing information programmed by the pilots.

Idle descent in many jets is around 3,000 feet per minute until reaching 10,000 feet. There is a speed restriction of 250 knots below 10,000 feet, therefore the flight management computer will slow the aircraft to 250 knots and continue the descent at approximately 1,500 feet per minute. Approximately 10 to 15 miles from the runway, the airplane is slowed to landing speed. Slats and flaps are extended passing 5 miles, at which point the airplane is at its approach speed on the lateral and vertical path to the runway.

John Cox is a retired airline captain with US Airways and runs his own aviation safety consulting company, Safety Operating Systems.