John Cox, special for USA TODAY| USATODAY
Question: I have noticed that every commercial aircraft takeoff, from throttle-up to wheels off the runway, takes about 30-35 seconds regardless of the size or loading of the aircraft. Is this because of some thrust-to-weight ratio requirement? Are the takeoff speeds to break contact with the ground also nearly the same?
-- submitted by reader Bob Felton, Houston, Texas
Answer: You are correct that a twin-engine jet will have an average takeoff run of 30 – 35 seconds, good observation. It will vary depending on the altitude of the airport, the weight of the airplane and the outside temperature. The lift-off speed (VR) will also vary depending on the weight.
Jets are certified to continue takeoff if an engine fails after the decision speed (V1). With a twin-engine jet, this requires that the operating engine be powerful enough to allow the airplane to continue to accelerate and climb over obstructions safely. One consequence of this is a significant excess thrust when both engines are operating. So the thrust-to-weight ratio is higher for twin-engine aircraft than for three- or four-engine aircraft.
It is not uncommon for the ground run of a four-engine jet at heavy weights to exceed 50 seconds; this is due to the thrust-to-weight ratio being less. An engine failure on a four-engine jet only results in a 25% decrease in the overall thrust, where on a twin-engine jet that thrust reduction is 50%. In a four-engine jet with three operating engines, there is sufficient thrust to clear obstructions and continue the takeoff safely. This is the reason that the twin-engine jets' ground run takes less time than a four-engine jet.
On many takeoffs, maximum engine thrust is not needed or utilized. This lower takeoff thrust setting dramatically increases engine life and lowers operating cost. Careful calculations are performed by the pilots prior to each takeoff to determine the needed thrust to meet all the takeoff requirements. If a reduced thrust takeoff is being made, the ground run may be slightly longer.
An average commercial jet accelerates to between 120 and 140 knots prior to liftoff. To do this in 30 to 35 seconds requires a good sustained acceleration. This is something that pilots look for during a takeoff roll.
Q: I live in Frankfurt, Germany, and from the city center have a great view of aircraft on approach for landing. Looking out the window, about 50% of the aircraft have the landing gear lowered early. I tried to figure out a pattern with aircraft, airline, etc., but can't. What determines when the landing gear is lowered?
-- Rick, Frankfurt Germany
A: There are many variables involved in the timing of landing-gear extension. These include the airline's operating procedure, whether an instrument approach is necessary to land, the speed of the airplane, the altitude of the airplane and others.
As an example, the extra drag of the landing gear can increase the descent rate or help slow the airplane, so it may be extended earlier if the airplane has excess speed or altitude on approach.
When the landing gear is extended, there is extra drag causing extra fuel burn. Pilots fly as fuel efficiently as possible. Consequently they will leave the gear retracted as long as they can.
Each flight is different, resulting in the differences you see in the extension of the landing gear.
John Cox is a retired airline captain with U.S. Airways and runs his own aviation safety consulting company, Safety Operating Systems.
