Standard procedure is to leave landing gear retracted until reaching the Final Approach Fix, about 6 miles east of the runway threshold and 5 miles west of El Dorado Hills.
This
is a 757-200 freighter overflying the west ridge of El Dorado Hills
with its landing gear down. Flaps are at "flaps 1", the flap setting
whose main effect is to deploy the 757's full span leading edge slats
to their intermediate position. The left photo is an enlargement
cropped from the original photo, on the right. Note the illuminated landing light on the nose gear. This was taken at 10.2
megapixel resolution using lens zoomed to 200 mm (telephoto) focal
length. Foliage in the right photo is a treetop.
Why extend the landing gear early? Most transport aircraft mount their main landing light(s) on the nose gear, it's necessary to low the gear in order to show a light. The most common reason to show a landing light in daylight operations is to make the aircraft more easily visible to the pilot of another aircraft in the vicinity. This is for flight safety, making it easier for pilots to visually assure separation from other air traffic in their vicinity. (See the "Why would..." section below for addition reasons and details of this one.)
Why extend the landing gear early? Most transport aircraft mount their main landing light(s) on the nose gear, it's necessary to low the gear in order to show a light. The most common reason to show a landing light in daylight operations is to make the aircraft more easily visible to the pilot of another aircraft in the vicinity. This is for flight safety, making it easier for pilots to visually assure separation from other air traffic in their vicinity. (See the "Why would..." section below for addition reasons and details of this one.)
As photographer and observer of this approach, this writer judged its sound level to be similar to that of normal approaches by A300-600 and 767-200 freighters or very slightly quieter. This would be slightly louder than for normal gear-up approaches by 757-200 aircraft. Additional aerodynamic noise due to extended landing gear probably was in the range of 3 dB to 6 dB. The maximum noise level (Lmax) probably was at most about 70 dB, more likely was closer to 65 dB.
Noise from aerodynamic drag with various combinations of landing gear and flaps deployed is higher than for a clean airframe at the same flight speed. However, noise from each source is not simply additive due to the combined aerodynamic effects of gear, flaps, and slats. Detailed measurements by NASA on a DC-9-31 and a DC-10-10 showed that in some cases extending landing gear while slats and flaps were already extended reduced noise, instead of increasing it. Such effects are due to favorable aerodynamic interactions between gear and flaps in particular configurations.
From the NASA measurements it would be reasonable to summarize by saying that extending any one of these devices (landing gear, slats, or flaps) aerodynamic noise increases substantially, on the order of 10 dB. Extension of additional devices increases noise by a much smaller amount and may in fact not increase total noise. The NASA report notes that the aerodynamic devices (gear, slats, and flaps) whose extension increases noise act substantially as diffracted dipole sources, whose contribution to noise intensity is proportional to the fifth power of airspeed. This makes airspeed extremely important to noise levels; extension of (particularly) slats and flaps allows reducing airspeed, with a consequent very high influence on reducing aerodynamic noise.
Why would a pilot fly this part of the approach with gear down and minimum flaps?
 | What
may be the most common reason is flight safety, to reduce risk of
midair collisions. This is especially applicable in the presence of
VFR (Visual Flight Rules) traffic which is not in radio communication
with Air Traffic Control. Showing a landing light is very effective in
assisting other pilots' ability to see and avoid the aircraft on
approach. Most transport aircraft carry landing lights and taxi lights on their nose gear. Thus, if Air Traffic Control advises a freighter pilot of other nearby traffic the pilot may extend the landing gear in order to show a landing light. This photo shows the nose gear of a UPS 757, illustrating the UPS-specific configuration of landing and taxi lights.
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An aerodynamic reason to extend landing gear early is to add drag when a high descent rate is needed. Extending landing gear without a change in power increases descent rate by increasing aerodynamic parasite drag. Here are two situations that can produce need for a maximum descent rate:
- Air
Traffic Control is late in delivering clearance to descend, leaving the
aircraft at a very high altitude on approach. A rapid descent is needed
to land directly; if the aircraft arrives too high at the airport it
cannot land. In that case it's necessary to fly at least one circuit in
a holding pattern while descending to avoid a last-minute missed
approach that would require flying a go-around. This is by far the most likely reason in our area.
- The aircraft experiences exceptional lift (rising air) on approach. Normally this is a factor only for slower and lighter aircraft, but in extreme cases it could possibly affect transport class aircraft. Convective (thermal) lift is frequently 1,000 feet per minute or more in mountain areas. Orographic lift (upslope winds) can be strong in particular small areas. Lee wave lift, especially common in mountainous areas during the winter, can be more than 3,000 feet per minute.
