If an airplane's cruising altitude is below 10,000 feet, do the landing lights stay on or do they come off?

They can be turned off if the cruising altitude is below 10,000 feet but once the airplane enters terminal airspace or before during initial descent, the landing lights go on.

I live in line with an airport runway, My old farm was close in, but my new farm is about 35 miles out, and by chance, in line with the same runway..

On take-off, the llanding ights are always off below 10,000 feet, usually more like 5000 to 6000 feet AGL. (est. from the ground) Sometimes on inbound aircraft, the lights are turned on way out, for visual contact. I was in a tower some years ago, servicing equipment. I was a ramp equipment mechanic back in the early 80's. I heard the ATC's requesting aircraft to turn on landing lights on occasion.

On some aircraft, the lights must be mounted on the gear struts. The lights swing to point downwards as the gear retacts. I can't tell what aircraft, of course, because I can only see that effect at night. It is particularly apparent if there is a bit of haze in the air.

I am not sure what the rules are for commercial/airline flights. I fly privately and in my aviation world the landing light is used for landing, noting else. during the cruise portion of te flight the airplane has a strobe light and navigations light that are switched on but not the landing light.

BTW - I don't think a Q400 would ever cruise below 10 000.

There are NO rules about operating landing lights below 10,000 ft -

Some operators have a company policy to turn them on -

It may also be "below 18,000 feet" -

In USA, they often have company policies "below 18,000 for this, that and everything" -

In that country, maybe the chief pilot's lucky number is 18...!

ICAO has a recommendation (not a regulation) in Annex 2 -

Turn ON lights within 30 NM of departure or arrival aerodrome -

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Landing lights are pilot controlled lighting, and their use is NOT regulated by the FAA. Many airlines have their own policies on use of landing lights below 10,000 feet on approach, but there is no requirement that they be used at all.

Landing lights are turned off when the gear comes up. Other external lights we turn off at 10K, but that's just a thing and not an FAR. Generally outside the airport traffic area is good enough.

good luck with the landing lights shining into the clouds or a snow.

on the other hand, it is nice thing to do for a night VFR traffic to flash the landing lights if you know there's a opposing traffic.. just to make yourself more visible to them.

There is no regulation about it, but standard AIRLINE procedure is to turn on the landing lights and recognition lights unless the aircraft is in cloud.