What does this terminal forecast mean to you?

[Scott Dennstaedt, PhD]

So you wake up early and are going to be flying out of Lawton, Oklahoma (KLAW) this morning departing around 13Z and see the following terminal forecast.  What does the forecast shown in bold mean to your for a go/stay decision?

 

KLAW 031120Z 0312/0412 13006KT P6SM BKN250 WS010/22040KT

   FM031500 18015G24KT P6SM SCT100 BKN200

   FM032200 33012KT P6SM BKN100

 

Wait until after 15Z?  Why or why not?  

[N201MKTurbo]

Wind shear at 1000 ft 220 deg 40 KTS 

[ragedracer1977]

A 40 kt wind shear at 1000 AGL is pretty significant.

[milotron]

KLAW runways are 17/35. Presumably active would be 17.

It would make for an exciting cross wind turn or climb through 1000'!

[N201MKTurbo]

The current surface winds are fairly light. The wind shear shouldn’t cause you any grief our planes accelerate pretty fast. It is probably pretty bumpy around the sheer. I would climb at a bit higher airspeed than normal until I got through it.

The surface winds pick up quite a bit at 1500. I would rather have the gusty wind in the air than on the ground. 

[gsengle]

I’d depart, and be ready for it. Prob wouldn’t want to arrive in that however, in IMC, the approach would be a bear. Keep your speed up.


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[Piloto]

Windshear are short duration low altitude phenomenas and hard to  forecast. Best defense is to have doppler radar on the field or airborne. Location of the event is also critical. A windshear event on final approach or takeoff can bring an airliner down but not a Mooney. Since the 1990s all the airliners are required to be equipped with predictive windshear radars (RDR-4 doppler) vs the old reactive system (radio altimeter). 

José

[gsengle]

Windshear are short duration low altitude phenomenas and hard to  forecast. Best defense is to have doppler radar on the field or airborne. Location of the event is also critical. A windshear event on final approach or takeoff can bring an airliner down but not a Mooney. Since the 1990s all the airliners are required to be equipped with predictive windshear radars (RDR-4 doppler) vs the old reactive system (radio altimeter). 
José

Windshear can absolutely bring down a Mooney. A microburst even more so. But we are talking then LLWS (low level wind shear)


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[N201MKTurbo]

9 minutes ago, gsengle said:

 

Windshear can absolutely bring down a Mooney. A microburst even more so. But we are talking then LLWS (low level wind shear)


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The only one I can find is the Angel Fire accident. That place can have severe mountain wave turbulence right down to the ground.

[gsengle]

The only one I can find is the Angel Fire accident. That place can have severe mountain wave turbulence right down to the ground.

 

Any light aircraft can be brought down by LLWS or a microburst. Airliners tend to fly in worse weather though.

 

https://www.aopa.org/news-and-media/all-news/1997/september/pilot/wx-watch-shear-threats

 

 

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[Piloto]

Unlike airliners Mooneys are over 100 times lighter (less inertia) and have instant power (piston engines) for quick acceleration. Just be ready on the throttle for when airspeed and altitude drops.

During development and FAA certification of the RDR-4 radar we (Allied Signal) used a Convair 580 with Allison engines (constant speed) to overcome the in situ windshear events at less than 500ft. In situ windshear events were measured and confirmed by comparing IRS data with ADC data and correlating the RDR-4 doppler data. Hard to spot a windshear event, most of them happen outside of an airport area. Because these are short duration events by the time we reached them many were gone.

José

Edited November 3, 2018 by Piloto

[Bob_Belville]

2 minutes ago, Piloto said:

Unlike airliners Mooneys are over 100 times lighter (less inertia) and have instant power (piston engines) for quick acceleration. Just be ready on the throttle for when airspeed and altitude drops.

José

The original question concerned taking off. At 1000' we'd still be full throttle. The precaution called for would be to climb at a higher airspeed so that if the sheer caused a near instant drop in airspeed of 40 kts the plane would not stall before airspeed built back. 

[Piloto]

42 minutes ago, Bob_Belville said:

The original question concerned taking off. At 1000' we'd still be full throttle. The precaution called for would be to climb at a higher airspeed so that if the sheer caused a near instant drop in airspeed of 40 kts the plane would not stall before airspeed built back. 

Good tip. But the speed drop in small planes is of short duration because of the lighter inertial mass.

José

[gsengle]

Read the AOPA article.


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[jaylw314]

1 hour ago, Piloto said:

Unlike airliners Mooneys are over 100 times lighter (less inertia) and have instant power (piston engines) for quick acceleration. Just be ready on the throttle for when airspeed and altitude drops.

During development and FAA certification of the RDR-4 radar we (Allied Signal) used a Convair 580 with Allison engines (constant speed) to overcome the in situ windshear events at less than 500ft. In situ windshear events were measured and confirmed by comparing IRS data with ADC data and correlating the RDR-4 doppler data. Hard to spot a windshear event, most of them happen outside of an airport area. Because these are short duration events by the time we reached them many were gone.

José

If I understand correctly, radar would only be helpful if there was precipitation across the windshear region.  If there was no precip, it wouldn't see anything

[kortopates]

Windshear is actually pretty common in my SOCAL area when we are having Santa Ana winds in the late summer and fall. Although I can't say I ever recall seeing one forecasted as strong as 40 kts at 1000' before! That beats what see around here. But its most common in the early part of the day before the sun has done much heating and surface winds are still calm under an inversion layer. But as it heats up during the day we get more low level mixing and those calm surface winds are gone. 

In our GA community, it poses a bigger risk to a few pilots I know of that insist on using Vx as a climb speed for quite aways because they believe it will increase their chances of getting enough altitude closer to turn back in an engine out situation. I subscribe to the opposite approach of getting airspeed well above Vy soonest where I can see much better, keep the engine cool and have more than a second to react if I do loose the engine. And the engine out analysis shows that approach isn't reducing your chances of being able to turn back either.

But going back to this scenario, someone mentioned surface winds would favor taking off on R17, and given the TAF, I would try for a right turnout departure into the winds at 1000' rather than a left turn out which would end up being downwind. Also as Bob mentioned above, be sure to have plenty of speed before reaching 1000'.

[kortopates]

8 minutes ago, jaylw314 said:

If I understand correctly, radar would only be helpful if there was precipitation across the windshear region.  If there was no precip, it wouldn't see anything

I don't think that is true as long as their is airborne dust or other particulate in the air and urban surface air probably always has enough to do so. I am no authority on this, but also remember that they have clear air mode versus precipitation modes for running the radar sweeps in order too pick up smaller particle returns (if I recall that properly).

[Piloto]

20 minutes ago, jaylw314 said:

If I understand correctly, radar would only be helpful if there was precipitation across the windshear region.  If there was no precip, it wouldn't see anything

On most windshear events there is some moisture or dust that the radar can detect. The radar is looking at the doppler spectrum to differentiate the windshear event vs rainfall or dust. At some airports highway traffic nearby can be mistaken by windshear (first noticed at Heathrow apt.) but later radar do overcome this false warning by scanning above and correlating.

 

José

Edited November 3, 2018 by Piloto

[Jim Peace]

2 hours ago, Piloto said:

 A windshear event on final approach or takeoff can bring an airliner down but not a Mooney.

Huh?  WTF?

[Piloto]

16 minutes ago, Jim Peace said:

Huh?  WTF?

That is why the FAA does not require Mooneys to be equipped with windshear radar.

José

[Scott Dennstaedt, PhD]

1 hour ago, kortopates said:

I don't think that is true as long as their is airborne dust or other particulate in the air and urban surface air probably always has enough to do so. I am no authority on this, but also remember that they have clear air mode versus precipitation modes for running the radar sweeps in order too pick up smaller particle returns (if I recall that properly).

In this case, the atmosphere was severe clear (see below) which is one of the catalysts to create this phenomenon.  The VAD Wind Profiles (VWF) from the NEXRAD Doppler radars often have high root mean square errors (RMSE) in these cases which is inversely proportional to the reliability of the data at that point. 

KLAW 031253Z 14006KT 10SM CLR 08/06 A2975 RMK AO2 SLP073 T00830061

For example, here's the VWP for the FDR radar  which is about 35 miles away from Lawton.  On the lower right, the mode (Md) is clear air and the max (Mx) wind is 52 knots.  Most of the wind barbs are in the 0 to 8 knot RMSE range (cyan) at 2,000 feet AGL.  Anything over 9.7 kts is considered unreliable.  So the winds are showing here as 52 knots from the southwest at around 13Z and they are fairly reliable.

 

[Scott Dennstaedt, PhD]

2 hours ago, Bob_Belville said:

The original question concerned taking off. At 1000' we'd still be full throttle. The precaution called for would be to climb at a higher airspeed so that if the sheer caused a near instant drop in airspeed of 40 kts the plane would not stall before airspeed built back. 

In this case Bob, you' should be preparing for smooth air.  If you were departing to the southwest, for example, the only thing you'll see on your instruments is that your ground speed would be dropping almost 40 knots during the initial climb.   But glassy smooth air.

[Scott Dennstaedt, PhD]

 

3 hours ago, N201MKTurbo said:

Wind shear at 1000 ft 220 deg 40 KTS 

Yes, but what does that mean to a pilot?

[Scott Dennstaedt, PhD]

3 hours ago, Piloto said:

Windshear are short duration low altitude phenomenas and hard to  forecast. 

José

That's not what this forecast that I posted is suggesting, however.  The phenomenon I posted is called nonconvective LLWS and is quite easy to forecast.  

[Scott Dennstaedt, PhD]

3 hours ago, N201MKTurbo said:

The current surface winds are fairly light. The wind shear shouldn’t cause you any grief our planes accelerate pretty fast. It is probably pretty bumpy around the sheer. I would climb at a bit higher airspeed than normal until I got through it.

The surface winds pick up quite a bit at 1500. I would rather have the gusty wind in the air than on the ground. 

Great observations.  But there won't be any bumps...in fact, it'll likely be glassy smooth.