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Years ago I was on a flight from San Francisco to Los Angeles, a 737 if I remember rightly, and was sitting just behind the wings and could not help noticing the flaps were partly extended for the whole flight.

I thought they looked odd out the window but did not bother mentioning it to the cabin crew since I was not sure what I was seeing. However, on landing in LA, the pilot retracted the flaps after using them for landing, and they were then correctly seated.

I assume the pilot felt something was off, since after landing I could see ground crew wandering around and inspecting the aircraft with puzzled looks on their faces.

I understand that the flap controls are a bit out of the pilots normal sight lines and this got me wondering whether there is instrumentation in the cockpit that alerts the pilot to the fact that the flaps are not fully closed once in normal cruise attitude. Perhaps even an indicator similar to the landing gear lights.

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It could have been as you witnessed.

In 2005, a modification package dubbed Quiet Wing was offered for the 737-200 and the 737 Classic (-3/4/500) that would droop the flaps by 4° for all phases of flight.

[It] decreases drag for cruise, and increases the aerodynamic efficiency of the wing by between 2% and 3%.

The NTSB in 1981 also found that many 727 pilots were doing the same thing in cruise—but manually and unapproved. They would pull a CB to keep the slats on the leading edge of the wing retracted, while the flaps extended to the their first stop.

Investigators believed that 727 pilots (in general, and this flight specifically) were setting the trailing edge flaps to two degrees during high altitude cruise, while at the same time pulling the circuit breaker for the slats so that they would not activate. This configuration was rumored to result in increased lift with no increase in drag, thus allowing more speed, higher elevation, or decreased fuel consumption. Flaps and Slats were intended to only be deployed at low speeds during take-off or landing.

To answer the title question, yes. Flap position indicators exist, and combined with the electronic speed tape, the maximum speed indicated will be slower.

If the flaps were to be extended at above design speed, an overspeed warning will be triggered as well.

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  • $\begingroup$ hmmm that's interesting, and it's so long ago it may even have been a 727 I was on. $\endgroup$ – Trevor_G Sep 15 '17 at 18:33
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    $\begingroup$ If it has no real down side, I wonder why they don't choose to simply integrate this change into the wing itself, and not have it as a flap setting. $\endgroup$ – Davidw Sep 16 '17 at 2:57
  • $\begingroup$ @Davidw as with most things it was probably cost. Once they realised an extra 2 degrees was beneficial, it's cheaper and easier to just use flaps rather than modify the wind design. $\endgroup$ – Notts90 Sep 16 '17 at 8:31
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There is a position indicator that shows what position the flaps are in. If they are in something other than "up", then the green "LE Flaps Ext" light would also be illuminated.

Photo of 737 Flap Position Indicator

That's not to say that those indications might not be missed.

There are a couple of other things that would be not-quite-normal (i.e. VNAV information would be unusual, commanding a speed no faster than the speed limit for the current flap configuration), but if you're missing the one indication, you might well misinterpret the others.

That said, there is no aural warning for the flaps being oversped the way that there is for an MMO or VMO overspeed (the clacker) or an approach to stall (the stick shaker). The only warning per se would be on the speed tape on a PFD or EFIS display or in the HUD. And in a Classic (non-EFIS) 737 without the HUD in use, those wouldn't be seen.

The most obvious indications would be the leading edge flaps and leading edge slats rattling around, and the hit on performance during climb and cruise (the leading edge devices are a lot of drag). Generally that sort of thing is pretty hard to miss, but again, if you're having the kind of day where you're missing the call to retract the flaps and missing the position indicator & its green light, you might miss the others too.

My first reaction on reading that account was, wow that crew messed up big! On the other hand, the "quiet wing" mod mentioned in the other answer makes very good sense as an explanation -- especially if the OP does NOT remember the buffet from the leading edge devices banging around. Let's go with that one!

By 2005, I'd hope that nobody was pulling CB's in order to extend the trailing edge flaps only -- the cautionary tale of what happened to the 727 whose crew did that was pretty well known by then. (Essentially, somebody who wasn't "in" on the plan saw the CB out & pushed it in, extending the leading edge devices at high speed. One of them departed the aircraft, and the it took tens of thousands of feet, IIRC, to regain control of the aircraft. Bad scene!)

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    $\begingroup$ "the aircraft spiraled out of control, diving about 34,000 feet (10,000 m) in just 63 seconds" - Yikes! Good thing they were cruising at 39,000, I guess. 34,000 ft in just over a minute is quite the descent rate. $\endgroup$ – reirab Sep 16 '17 at 0:46
  • $\begingroup$ That's a very significant fraction of cruising speed...straight down! Are you sure that's correct? $\endgroup$ – AaronD Sep 16 '17 at 1:01
  • $\begingroup$ @AaronD Since "out of control" can mean "you just stopped flying and the plane is now a falling brick" that descent rate isn't impossible. If you happen to be falling nose first, the engines are also helping you hit the ground sooner, of course. $\endgroup$ – alephzero Sep 16 '17 at 2:26
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    $\begingroup$ @alephzero I can believe that that speed could be reached, but I'm thinking also about the implied recovery. (or maybe I'm combining two incidents) What kind of acceleration does it take to cover that distance in that time, being vertically stationary at both ends of the ordeal? And is it survivable? $\endgroup$ – AaronD Sep 16 '17 at 2:57
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    $\begingroup$ @AaronD - assuming equal acceleration and deceleration, that means 5000m in 31.5s in each half of the movement. With zero initial vertical motion, that means acceleration is 2*5000/31.5/31.5 = about 10m/s/s, or roughly one g. Uncomfortable, but definitely survivable. Looked at another way, this is basically the trajectory the vomit comet follows, albeit maintained for a little longer time than usual. $\endgroup$ – Jules Sep 16 '17 at 9:00

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