I live near Moffett Field, and I noticed that some airplanes go over the field once before landing, along the runway (not across), even when the tower is open. I thought that was a procedure for non-towered airports only?
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Moffett Field is a federal airport (requires permission) owned by NASA and has a wide ranging number of traffic patterns/approaches available. In answer to your question, you may be seeing what is called an "Overhead" approach. This is a VFR maneuver quite common at military bases and usually consists of the following:
- Airplane approaches the runway in use at an altitude well above the airport (2000 ft AGL or higher). Usually this initial altitude is higher than the downwind altitude.
- At some point when over the runway (over the numbers, mid-field, departure end, etc.), the pilot will break left or right and enter a descending turn that will include a modified downwind length based on the point at which the overhead break occurred.
- The pilot will then continue to a base leg and ultimately a final approach (often very short) to a landing or touch and go.
- This maneuver is done under VFR as coordinated with, and cleared by, the Tower.
This type of maneuver could explain, based on your question, what you are seeing at Moffett Field. See the pictures below.
From the Moffett Airfield Operations Manual:
Example Overhead approach: (just an example, many variations are possible)
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$\begingroup$ Yes, the traffic is mostly military, NASA or gov related. The small airplanes use the IFR approach sometimes for practice but then break off the approach and go somewhere else (I'm assuming they practice e.g. the ILS). $\endgroup$– FrankCommented Feb 28, 2018 at 22:11
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$\begingroup$ What are the reasons to do this approach rather than a straight in? The military jets seem to prefer the overhead approach, but the C-130s and other government jets usually land straight in. There presumably are not many surprises on the runway that you should check before landing, Moffett is controlled and very civilized... $\endgroup$– FrankCommented Mar 1, 2018 at 0:11
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3$\begingroup$ @frank - one of the primary reasons is that very often military aircraft fly in formations (2 or more) and flying an overhead pattern allows the aircraft, once over the airport, to "break" a few seconds apart thereby easily establishing an in-trail downwind entry. Also, it allows for aircraft to fly at a fairly high rate of speed to the airport and quickly dissipate the speed (extend gear/flaps/spoilers) during the maneuver. Probably some other reasons, but it's very common in the military. $\endgroup$– user22445Commented Mar 1, 2018 at 0:25