View Full Version : Effects of Turbulence Upon Airspeed and Hazards to Aviation Assoicated with CBs

21st Feb 2015, 22:30
I've started this thread to open a discussion about what may be an under-appreciation of thunderstorms and their effects on safety in aviation.

Recent studies have even emphasized the importance of re-reviewing FAA guidelines for thunderstorm avoidance.

It may be that if the effects of turbulence upon airspeed are better understood, accidents can be prevented.

I'll start this discussion with an animation showing, at 30 second intervals over a 21 minute period of time, the intense windshear layers which form near, around, and over the top of thunderstorms as the result of convection inducted turbulence (CIT)

Credit: Turbulence above thunderstorms
(https://avwxworkshops.com/etips/11-12-10.html)The blue areas in this diagram are cloud water content (above some threshold) and the red areas are model-produced turbulence (actually referred to as subgrid turbulence kinetic energy).https://avwxworkshops.com/etips/images/CIT-Simulation.gif

Its the kinetic energy part I'll discuss below. First, some background on what the above image shows:

Scientists at the Research Applications Laboratory (RAL) have investigated aircraft encounters with turbulence above thunderstorms and find that the FAA guidance is a bit naive. One of these incidents occurred on 10 July 1997 over Dickinson, North Dakota. A commercial turbojet aircraft encountered severe turbulence as it was negotiating a path through a number of scattered thunderstorms. At the time of the encounter it was passing directly over a developing deep convective cloud. In this incident, 22 passengers sustained minor injuries and the aircraft sustained enough damage to cause it to make a precautionary landing at Denver, Colorado. This aircraft encountered what is referred to as convection induced turbulence (CIT)." This type of turbulence is common enough that the CIT acronym is well known within the commercial aviation community; it is used to describe turbulence in the clear air either above the thunderstorm top, under the anvil, or near the lateral visible boundaries. The aircraft's transit of the storm:


The cauldren of turbulent air it overflew:


Source: AMERICAN METEOROLOGICAL SOCIETY Bulletin: Recent advances in the understanding of near-cloud turbulence 2/28/11 (http://web.atmos.ucla.edu/~fovell/papers/2012-Lane-etal.pdf)

Regardless of the source dynamics, the spatial scale of the lateral bands and downstream structures are all approaching those that strongly influence commercial aircraft and they may pose an additional hazard adjacent to the storm and be responsible for the known hazard downwind.

5. Summary and future outlook: Fundamental understanding of near-cloud turbulence (NCT).

We have summarized recent progress made in understanding the NCT aviation hazard. These fundamental advances in our basic understanding were enabled by high-resolution
numerical simulations of observed events, complemented by improved data on turbulence encounters. These studies have shown that NCT is a complex phenomenon that crucially
depends on cloud characteristics, the structure of the near-cloud environment, and perturbations to that environment by cloud circulations and gravity waves. Yet, we are
acutely aware that we are only beginning to scratch the surface and a variety of basic problems are still to be solved regarding the dynamics underlying NCT. Outstanding
questions include:

--- What are the characteristics of NCT and how does it vary spatially and temporally?
--- How is NCT related to the mode of convective organization and its intensity?
--- What is the relative importance of gravity wave breaking and Kelvin-Helmholtz instability to the turbulence hazard?
--- What are the processes leading to the enhanced hazard near thunderstorm anvils?
--- What is the structure and mechanism of turbulence in thunderstorm wakes?
--- How common is the hazard posed by turbulence associated with ducted gravity waves?
--- What is the relationship between observable cloud features, the mesoscale environment, and NCT that may be useful for pilots and aviation forecasters?
--- What is the climatology of NCT?

Answering these questions and further research on the other processes detailed in this article is necessary to advance the fundamental understanding of NCT and could also
provide the framework to develop new approaches for turbulence avoidance. With recent improvements in high-resolution modeling capabilities available to researchers, we
believe that such much-needed advancement is achievable. Unfortunately, despite the importance of this problem and the opportunities for progress, there is relatively limited
activity in this area with only a few groups around the world actively engaged in NCT research. We hope that this article has spurred additional interest in this topic and we
encourage others to study this challenging problem.
I'll admit I'm not scientist, but I'd like to touch on a few things I have some personal experience with concerning the effects upon airspeed (and instantaneous leaps in speeds) associated with aircraft repeatedly crossing shear layers / boundary layers, below.

First, some statistics:


Source: Statistical Summary of Commercial Jet Airplane Accidents Worldwide 1959 - 2006 (http://www.skybrary.aero/bookshelf/books/2729.pd)

An FAA Advisory Circular (http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCAQFjAA&url=http%3A%2F%2Fwww.faa.gov%2Fregulations_policies%2Fadviso ry_circulars%2Findex.cfm%2Fgo%2Fdocument.information%2Fdocum entID%2F22291&ei=eQXpVNLDHNChoQTm84IQ&usg=AFQjCNGXvmiz9VAFW6e3B5UAV5dhRZ0_iA&sig2=nxrsEkwP9bdHFJ8gOd43Bw): Pilot Windshear Guide (11/25/88)



22nd Feb 2015, 07:45
--- What is the relative importance of gravity wave breaking and Kelvin-Helmholtz instability to the turbulence hazard?
--- What are the processes leading to the enhanced hazard near thunderstorm anvils?
--- What is the structure and mechanism of turbulence in thunderstorm wakes?From the animations above, its clear that shear layers of rapidly changing airspeeds, pressures, intensity and direction co-exist with the underlying structures of thunderstorms. Thunderstorm like those encountered by a growing list of aircraft that have crashed when meeting them (http://avherald.com/h?search_term=thunderstorm&opt=7680&dosearch=1&search.x=17&search.y=14) including

Malaysia QZ-8501




Incredible airspeed changes are possible when an airframe crosses boundary layers. By repeatedly crossing boundary layers between areas of high windspeed and low windspeed (and high pressure/low pressure zones), a kinetic transfer of airspeed gains are realized, and have been looked at by NASA (http://www.buscarvideoshd.com/nasa-albatross-dynamic-soaring-open-ocean-persistent-platform-uav-concept/F4zEaYl01Uw.html) in research into perpetual flight, similar to the Albatross's seeming ability to cross oceans.

In Dynamic Soaring, a 3 Meter remote control glider has now been flown to 505 miles per hour (http://youtu.be/hFPJ6DUAY10) -- airspeeds near the maximum airspeed attainable by an Airbus A320. Purely by repeatedly crossing boundary layers / shear layers, like the ones in the animation above, and gaining airspeed with each crossing.

High altitude windshear has similar effects and has the potential to overspeed an aircraft at cruise in one moment and then plunge it into a void of downwards stalling air the next moment, and repeat that process very quickly, confusing pilots, and taking auto-pilots offline.

AIRBUS SAFETY LIBRARY (http://www.airbus.com/fileadmin/media_gallery/files/safety_library_items/AirbusSafetyLib_-FLT_OPS-ADV_WX-SEQ02.pdf)
II.2 Defining Windshear

Windshear is defined as a sudden change of wind velocity and/or direction.
Windshear occurs in all directions, but for convenience, it is measured along vertical and horizontal axis, thus becoming vertical and horizontal windshear:

Vertical windshear:
−Variations of the horizontal wind component along the vertical axis, resulting in turbulence that may affect the aircraft airspeed when climbing or descending through the windshear layer
−Variations of the wind component of 20 kt per 1000 ft to 30 kt per 1000 ft are typical values, but a vertical windshear may reach up to 10 kt per 100 ft.

Windshear conditions usually are associated with the following weather situations:
•Jet streams
•Mountain waves
•Frontal surfaces
•Thunderstorms and convective clouds
Influence of Windshear on Aircraft Performance (http://www.airbus.com/fileadmin/media_gallery/files/safety_library_items/AirbusSafetyLib_-FLT_OPS-ADV_WX-SEQ02.pdf#page=3&zoom=auto,-274,614) The flight performance is affected as:
• Headwind gust instantaneously increases the aircraft speed and thus tends to make the aircraft fly above intended path and/or accelerate

• A downdraft affects both the aircraft Angle-Of-Attack (AOA), that increases, and the aircraft path since it makes the aircraft sink

• Tailwind gust instantaneously decreases the aircraft speed and thus tends to make the aircraft fly below intended path and/or decelerate.I believe the effects upon airframes of thunderstorms and shear layers are not well enough understood and have played large roles in recent hull losses and life.

AF447's encounter with a the kinetic energy of a thunderstorm's 'turbulence':


Yemenia A310 Flight 626 (http://en.wikipedia.org/wiki/Yemenia_Flight_626)


A320 Alpha Floor Activation followed by initial overspeed and subsequent stall (http://www.atsb.gov.au/publications/investigation_reports/2014/aair/ao-2014-049.aspx)


The Pilot of this aircraft said :


I agree with the scientists studying these factors, that more research is needed.

22nd Feb 2015, 17:51
Excellent post with supporting data on the subject

23rd Feb 2015, 07:46
A superior study of windshear. As pilots, we would normally fly on our experience in these matters. The problem is that commercial considerations are thrust on us,
and sometimes we go like lambs to the slaughter.

Keeping a schedule plus CFP calculated fuel to reduce costs are a perfect storm when there are perfect storms.

All that Cb and windshear knowledge becomes less significant if crews cannot consider safer options before departure or perhaps before the previous refuelling opportunity.

CRM principles are no less applicable to TS than an all engine out ditching in the Hudson. I consider that weather considerations should be an advanced segment of CRM training.