February 19, 2014

The Terrifying Turbulent Flight of United 1676: Why did it happen?

 There is the potential for lowland snow over the weekend...will have a blog dealing with this issue later in the afternoon.  A LOT of uncertainty about it...cliff

Announcement:  The Northwest Weather Workshop will be on Feb 28th/March 1st in Seattle.

The NW Weather Workshop is the big annual gathering of those interested in the weather of the Pacific Northwest and everyone is welcome.  For more information, including the agenda and registration information, please check out: https://www.atmos.washington.edu/pnww/

United Flight 1676, en route from Denver to Billings Montana, were cruising at 34,000 ft above Wyoming around 3 PM Mountain Standard Time (MST) Monday when the severe turbulence hit the plane.

Passenger heard a loud boom, the  Boeing 737 aircraft tilted to the right and plummeted downward for several seconds.   Unbelted passengers and crew hit the ceiling, a baby was pulled form a parent's arm, and several passengers were seriously injured, one critically.  No warning was given prior to the encounter and the plane landed in Billings less than a half hour later.

Why did this incident occur?  Is their any way to avoid such serious events?  Such are the topics to be explored in this blog.

According to FlightAware, the aircraft left Denver at 12:12 PM MST and landed in Billings at 1:25 PM MST.  The route crossed Wyoming from the SE to NW and no thunderstorms were evident on the radar (see image).

This flight track took the aircraft over the terrain of the Bighorn National Forecast and east of the more substantial terrain surrounding Yellowstone National Park (see graphic).

The severe turbulence hit the plane around 1 PM  MST (3 PM EST) when the aircraft was over northern Wyoming.  Around this period the plane experienced a sudden loss of speed and a drop in altitude (see plot from FlightAware below).

Several aircraft in the area on Monday afternoon reported moderate to severe turbulence:  here are some sample pilot reports documenting this fact:

GCC UUA /OV CZI245027/TM 2036/FL380/TP B737/TB SEV 380-340/ZLC

TB SEV means severe turbulence. Altitudes between 34,000 and 38,000 ft.

Since there were no thunderstorms in the area, the number one candidate for producing this turbulence was high amplitude breaking mountain waves.

In a stable atmosphere (where the air want to return to its original level when pushed up or down), mountain waves often form when air is forced upwards by a terrain feature (see image)

High amplitude mountain waves occur when strong winds approach mountain accompanied by the proper changes of stability (related to temperature) and wind with height.  Such waves can amplify to the point that they break, producing severe to extreme turbulence (see figure below).  The turbulence from breaking mountain waves can be highly localized in altitude and position, with change of only a few thousand feet or a few miles, taking an aircraft from extreme turbulence to a smooth ride.  Turbulence can also be highly transient, developing or disappearing over a few minutes or an hour.

Let's check model forecasts for this time to see whether they predicted such mountain waves.  Here is the UW WRF forecast (12-km domain, 7hr forecast) for noon MST showing the winds and vertical motions (colors) at 500 hPa (around 18,000 ft).  You see the strongly varying colors over Wyoming?,with  alternating bands (red to blue) --those are couplets of upward and downward motion associated with high amplitude mountain waves.

And take a look at the 1 hr forecast for maximum updraft velocity  in the vertical air column for the same time from the NOAA/NWS HRRR model (3 km resolution).  You see the light blue band over north central Wyoming.   The HRRR had the mountains waves as well.

And now another tool: high-resolution MODIS satellite water vapor image provided the University of Wisconsin (CIMSS).  Where the air is descending there is less water vapor (dark purple), rising--more water vapor (lighter colors).  You see the striations near the Bighorm Mountains....that is probably the signature of the mountain waves.

The National Transportation Safety Board, the FAA, and United Airlines will undoubtedly be investigating this turbulence event in more detail, but it sure looks like the result of the plane heading into a breaking high-amplitude mountain wave.

As our forecast models continue to our improve and satellite imagery gets more sophisticated, perhaps one-day aircraft will preemptively avoid such locations.  If you really don't want to risk getting into such situations, by watching the models and cancelling flights on days with strong mountain wave activity you would escape most of them.  The lee (eastern) side of the Rockies is notorious for mountain waves, so avoid airports like Denver and Billings if you really dislike mountain wave turbulence.


  1. WHY were some assengers unbelted? Don't flight attendants check that kind of thing?

  2. FAA definition of severe turbulence ...

    Large, abrupt changes in altitude and/or attitude. Usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control. Occupants are forced violently against seatbelts. Unsecured objects are tossed about. Food service and walking are impossible.


  3. "WHY were some assengers unbelted? Don't flight attendants check that kind of thing?"
    Humans are dumb.

    Ok, a better answer. There's a small handful of attendants on any flight. Yeah, they do a pre-flight check, but people unbuckle as soon as they get the check out of the way, or forget to buckle up again when they're coming back from the bathroom, etc. Barring an electronic buckle monitor, there's no way for the attendants to keep everybody buckled up.

    (Not an attendant; I'm just a passenger and an observer of human behavior)

  4. At the same time as this event, a glider flew from Minden, NV to Casper, WY using the same mountain wave conditions. The flight was over 940 st.mi., in just over 10 hrs. They will be claiming a new US distance record for multiplace gliders. They were lower, 12K' to 26K'.
    A retired NWS forecaster in Cal. was their ground support for the flight and has some info on his blog:

  5. People that don't buckle there seatbelt on flights are not only dangerous to themselves, they are dangerous to other people, as they literally can turn into human missiles during extreme turbulance.

    Do what the Pilot's do: when you're seated, which should be the vast majority of the time, buckle up.

    Nobody is saying you can't get up for a minute to stretch your legs or take a pee, but buckle up when you sit back down.

  6. As a glider pilot, I enjoyed wave flying, but never enjoyed encountering a rotor cloud (or rotor without cloud!). Extremely violent and dangerous because the rotor is usually quite low. My most memorable rotor encounter was in Vermont at pattern altitude. Complete loss of control and the stick grip came off in my hand. G meter recorded plus 6, minus 4 g's.

  7. Hi Cliff,

    I'd love to hear your take on the potential for snow this weekend. It sounds like us Whatcom County folks make get in on the snow action this time :-)

  8. A good analysis, Cliff. This sort of turbulence is a great argument for the following:
    - assume clear-air turbulence is ALWAYS possible and always wear your seatbelt low and snug when seated
    - buy your baby a seat and put them in their FAA-approved five-point harness carseat on the plane.
    - Flying into/out of any Montana location is a pretty good bet for some turbulence if you enjoy the rollercoaster :-)

  9. I would think, as Dr Mass pointed out, as modeling improves high turbulence areas should be better predicted. Airlines can then alter their flight plans.

    It's all about data and computing/modeling power.

  10. We encountered a Mountain Wave on a AK flight from NY to Seattle last May (during the week of the tornados in Moore, OK) - in the same approximate longitude but over Montana. It was very sudden, violent & scary, but the pilot said the plane was built to handle it but good reason to stay fastened.


  11. >>>Why were some passengers unbelted?<<<

    Because many people don't bother to buckle up, or they do so lazily, keeping the belt very loose and effectively useless.

    You might wish to read my take on these turbulence incidents here...



  12. Cliff: I've always thought that severe mountain waves form northeast of Mt. Rainier more often than recognized, and that several general aviation accidents might be attributable to these even though not recognized by the NTSB. Do you know if anyone has studied this?


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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