Ask the Captain: Why does the descent take so long?

Question: Aircraft achieve altitude fairly quickly on takeoff, so why the long, slow descent for landing?

— Submitted by reader Dave Shepherd, Houston

Answer: In most cases, air-traffic control instructs the pilots to begin their descent. Often it is earlier than the optimum distance but necessary to get the aircraft in line with traffic ahead.

Q: I fly in and out of Chicago often. Every landing seems to take forever. As an example, we start our approach at 200 or more miles out (as compared to say 150 miles anywhere else). Is this caused by heavy air traffic or other restrictions?

— Doug, Manchester, N.H.

A: In high-density areas such as Chicago, the descents can begin many miles from the destination. This is done to smooth the arrival and departure flows. One of the benefits of the upcoming next-generation air-traffic control system will be better descent planning using modern technology for high-density areas. This will help with fuel efficiency and flight times.

Q: I was a passenger on a Western Airlines DC-10 in 1983. The flight was en route to Anchorage when the pilot announced, "We'll be doing an accelerated descent, and we'll be on the ground in 10 minutes." The plane went into a steep dive with the wing flaps out. Can you explain what controls were used and how the pilot did this maneuver?

— Ben McKernan, Portland, Ore.

A: The "flaps" you saw were flight spoilers. These panels on top of the wing decrease lift, which increases the rate of descent. They also add drag, keeping the speed down.

An emergency descent is a maneuver practiced by pilots during training. This maneuver is used when it is necessary to descend as quickly as possible, such as during a pressurization problem or if there is a medical emergency requiring a diversion.

Q: Is there a certain angle used to land a plane safely? Does it vary when various flight controls are in use?

— James Adams, Las Vegas

A: The normal descent profile is approximately 3 degrees. This can vary, but during the final stages of landing, 3 degrees is usually the target. Wind can vary the groundspeed and descent rate, but the descent angle remains the same.

Decelerative devices do not change the approach angle.

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