How does the pilot know there is turbulence ahead?

[Warks]

Flying back home LAX-SYD a couple of weeks ago and the "Fasten seatbelts" light was turned on in the dead of night (somewhere around the ITCZ no doubt). From experience I've found they tend not to do this if it's just a bit of wobble-wobble but something a bit bigger. I wasn't disappointed with some real rough and tumble for what seemed like 5 minutes but was probably only 40 seconds.
Then it was all smooth flying again and the light went off for the remainder of the trip until descent. It made me wonder how they knew or could detect what was obviously only a small patch of turbulence like that. I can understand that they can see big storms and the like on the radar but relatively small patches? What would it look like?
Anyway it piqued my interest.

 

[serfty]

There was possibly another aircraft on the same flight path (UA?) 10 minutes ahead ...

 

[harvyk]

The other possibility is Weather radar... Not sure of the exact limitations of the systems, but they can pick up turbulence from quite a distance away, they can also sometimes pick up very small patches of turbulence.

Also remember that turbulence is not a static thing, it can and does move around in the air (by its very nature), thus a patch of turbulence may have looked far bigger or worse on the screen, but by the time the aircraft actually reached that area it had dissipated itself somewhat.

 

[straitman]

The other possibility is Weather radar... Not sure of the exact limitations of the systems, but they can pick up turbulence from quite a distance away, they can also sometimes pick up very small patches of turbulence.

Also remember that turbulence is not a static thing, it can and does move around in the air (by its very nature), thus a patch of turbulence may have looked far bigger or worse on the screen, but by the time the aircraft actually reached that area it had dissipated itself somewhat.

Weather radar will not pick up turbulence alone but it will show any weather. It is possibly what serfty said as any significant encounters are passed to other traffic or there was some associated weather showing on the radar.

 

[harvyk]

Weather radar will not pick up turbulence alone but it will show any weather. It is possibly what serfty said as any significant encounters are passed to other traffic or there was some associated weather showing on the radar.

True, sorry I should have said bad weather associated with turbulence... (from memory it's the water droplets in the air it sees)

 

[straitman]

True, sorry I should have said bad weather associated with turbulence... (from memory it's the water droplets in the air it sees)

Yep :!: Sure is.

 

[markis10]

Both Delta and Southwest are trialling EDR (eddy dissipation rate) equipment in their 737 fleet which uses existing weather radar systems to determine vertical wind currents, and provides reports back to the ops centre to help with maintenance on aircraft post turbulence, as pilot reports are historically infrequent by comparison to such automated systems. Such a system may give advance warnings on storm based turbulence, however for clear air turbulence (CAT) its a lot harder, generally gorund based systems such as doppler lidar may detect the occurance of CAT and a warning given to the pilot, otherwise its a case of first one in warns the others!

The possibility of CAT in a given area will be warned via a SIGMET (significant meterological event ) notice, kind of like a NOTAM to do with weather, and often pilots will request a buckle down when passing through the areas for the first time, interstingly there is a SIGMET current for turbulence in the Melbourne flight information area at present for sub 8000 ft:

SIGMET WSAU21 AMRF 020254
YMMM SIGMET ML01 VALID 020500/020900 YMRF-
YMMM MELBOURNE FIR SEV TURB FCST WI YBOR - YMBU - YKII - WEBS BLW
A080 MAINLY LEE OF RANGES STNR INTSF FROM W
STS:NEW


This is an example of one for higher fliers currently for the Brisbane FIR:

[FONT=TimesNewRoman,Bold]SIGMET WSAU21 AMMC 020559
YBBB SIGMET BT02 VALID 020626/021026 YMMC-
YBBB BRISBANE FIR SEV TURB FCST WI
S2300 E15100 - S2000 E15100 - S2100 E16300 -
S2500 E16300 -
FL240/360 MOV NE 10KT NC.
STS:REVIEW BT01 020226/020626=
[/FONT]​

 

[Cyclogenesis]

I used to be one of the forecasters responsible for issuing sigmets above FL100 for the Australia region of responsibility. In general an airliner will not enter an area where severe clear air turbulence has been forecast/observed. ATC will modify spacing to avoid that area. International significant weather charts will show areas of moderate clear air turb, and pilots have access and review these prior to take off.

When I worked on the bench I would issue warnings based on soundings (baloon launches) sattelite evidence and pilot reports.

These warnings are repeated through ATC and ATC is the primary source of clear air turbulence information..

Now on to my area of current area of expertese weather radar. Yes the primary scatterer for the X-band systems commonly installed in the nose of any major airliner are hydrometeor (ice, rain etc...) however you can see fluctuations in moisture content associated with CAT (called brag scattering) however I would assume most display systems would not be sophisticated enough and not expect pilots to know what to look for to dignose CAT in X-band signals... In summary the primary indicator of CAT is a mix of the SIGWX chart and pilot reports concatinated by ATC

 

[Mr Ed]

Too much info there and sounds a bit gobbily gock.

The pilot being at the front of the plane feels it first.

 

[markis10]

The pilot being at the front of the plane feels it first.

Not quite true in most circumstances, in fact with CAT its well known that the effects are felt first and then more pronounced from the wing backwards to the tail as the turbulance has its effect on the primary lift/drag areas that produce resistance to the change in force.

 

[Mr Ed]

Why does this place have so many aeronautical know it alls?

 

[Flashback]

Why does this place have so many aeronautical know it alls?

Do you want the obvious answer? No? Ok........ here it is ......... it's an aeronautical centric forum! :shock:

 

[Warks]

Thanks all, I think serfty's response makes sense in this case. It's a lonely old path across the Pacific except from about 10pm onwards!