Wing Cleaning
 

yesterday as we were about to push back on our SEA-DFW flight on a 738 the pilot came on to tell us the wings needed to be rinsed off (??). Sure enough a few minutes later a de-icing truck with scissor lift pulls up and starts to spray what appears to be water. FWIW it was in the mid 50's outside.

Anyone had this happen before? Did some Canadian honkers crap on his wing?

It never happens to me.

But FWIW - if the wings are dirty, the dirt can affect flight efficiency.

The plane gets cold-soaked at altitude, and picks up moisture on the descent. It freezes to the wings. Even though it is in the 50's on the ground, the ice/frost will not melt quickly, and must be removed prior to the next departure. Now you know.

That sounds like that could be 95% of every plane that lands so why doesn't this happen often?

planes do not always pick up moisture on the descent (fly through clouds that are high moisture). Or the amount of frost is minimal and melts quickly. Or the frost is not on critical surfaces (leading edge of wings or on tail)

I just find it amazing that for someone like me who is on a plane every week of the year this is the first time I've ever seen it. Go figure!

Maybe the pilot just wanted to fly a clean plane.

Nah, I don't think so. So in 50 years of flying this has never happened to me, yet you describe it as a normal occurrence? I expect that the more likely explanation would be a one-off, and might include things like an inadvertent chemical/fluid/fuel spill on one of the wings, pilot noticed something when doing his/her walk-around, etc.

Both of these things can be true. People always overestimate the degree to which their anecdotal experience reflects reality. :)

I recall reading about this in Patrick Smith's "Ask the Pilot" column some time ago. It happens.

I had this happen to me just once.

I did a MR on a LHR turn for EXP on Christmas Eve, with a Christmas Day return. I sat on the same plane in the same seat for both legs and had to wait on board in LHR for 45 minutes for de-icing.

The captains explanation was the same as nacosdelux's.

I've seen it happen in Phoenix a time or two. Except no deicing equipment so flights were delayed until the ice melted.

Captain explained that they were hoping that refueling would add enough warm fuel to de ice organically. No such luck. Had to wait for de icing equip,net which took a while. At PHX of all places.

While you are doing some cleaning, would you mind cleaning the seat back pocket in front of me as well .........

Here's a good, concise definition of cold soaking. Yes it's a real thing.

Edit: And here is a longer explanation. Cold soaking starts at bullet point No. 2.

Cold soaked fuel going into the wing sometimes causes frost to form. It’s fairly common to see this under the wing on a pre-flight walkaround which isn’t a big deal because that doesn’t provide lift.

However, frost on the top of the wing disrupts the airflow and does reduce lift. That’s why you want that cleaned off.