Turbulence Penetration Speed

- James Albright (a former G450 driver)

Updated: 2014-01-14

There are a series of compound lies in the G450 Airplane Flight Manual, Limitations, when it comes to speed, among those:

• VA Maneuvering Speed = 206 KCAS

• Turbulence Penetration Speed = 270 KCAS (at or above 10,000'), 240 KCAS (below 10,000')

But let say you believe both of those numbers. Why would you want to "penetrate" turbulence so much faster than maneuvering speed? Before you read further, discover why VA ≠ 206: G450 Maneuvering Speed.

Bottom line up top: If the ride gets a little rough, slowing to 270 or 240 KCAS will help. But if it is really turbulent, if you want to avoid exceeding airframe stress limits, the numbers will be considerably lower. The G450 Aircraft Operating Manual does offer a minimum speed of 1.5 VS to maintain adequate buffet margins and you have a ready display of 1.3 VS on your display controller, the line that says VREF. Add 15% to that number and you've got a good speed for when it gets really turbulent. Of course that is just my technique, you do what you think is safe.

Everything here is from the references shown below, with a few comments in an alternate color.

 14 CFR 25 Design Criteria

[14 CFR 25, §25.1517] A rough air speed, VRA, for use as the recommended turbulence penetration airspeed in §25.1585(a)(8), must be established, which—

(1) Is not greater than the design airspeed for maximum gust intensity, selected for VB; and

(2) Is not less than the minimum value of VB specified in §25.335(d); and

(3) Is sufficiently less than VMO to ensure that likely speed variation during rough air encounters will not cause the overs peed warning to operate too frequently. In the absence of a rational investigation substantiating the use of other values, VRA must be less than VMO—35 knots (TAS).

One of the references in this section of 14 CFR 25 does not exist and the other is fairly math intensive. You can skip the math because it can be broken down to the following:

• VRA is selected by the aircraft manufacturer.

• VRA must be chosen to provide "an optimum margin between low and high speed buffet boundaries.

• VRA need not be greater than design cruising speed.

So, just as with VA and VMCG, the aircraft manufacturer has wide latitude in coming up with VRA. The number satisfies the feds, but means very little to you, the pilot.

 Gulfstream Turbulence Penetration Speed

[G450 Aircraft Operating Manual, §1-03-40; ¶ 9] Turbulence Penetration Speed:

1. Altitude 10,000 Ft Or Above: 270 KCAS / 0.75 MT, whichever is less

2. Below 10,000 Ft: 240 KCAS

[G450 Aircraft Operating Manual, §07-02-30-03-40; ¶ 3.A.]

• Recommended turbulence penetration airspeed is 270 KCAS / 0.75 MT, whichever is lower.

• A minimum speed of 1.5 VS is recommended to maintain adequate buffet margins.

• Severe turbulence will cause large and often rapid variations in indicated airspeed.

There are two problems associated with turbulence, one far worse than the other: comfort and structural integrity. Experience tells us that the ride is better when slower, no doubt about it. But at what point is the structure of the airplane in jeopardy? The manual gives us a range of speeds: no lower than 1.5 VS, no higher than 270 KCAS or 0.75 MT. You should obviously endeavor to keep the airplane in air that only threatens comfort. But what about when it really gets rough? I think we can find a better speed than 270 KCAS.

 Gulfstream Maneuvering Speed — Gulfstream's Story

Ask just about any Gulfstream pilot for their maneuvering speed and, if they have an answer, they will probably say "206 knots." That's the book answer. Which book? The AFM for the GIII, GIV, GV, G350, G-400, G450, G500, and G550. Question: how can they all have the same VA when they vary greatly in gross weight? Answer: they don't.

Ask a G450 pilot what configuration, weight, or altitude that 206 knots applies to you will probably get this: "All of them." That isn't true either. The actual VA for most landing configurations is around 140 knots. At altitude the number can be much higher than the published 206 knots.

More about this: G450 VA - Maneuvering Speed.

If you are in truly turbulent air and want to avoid overstressing the aircraft, you need to be at VA maneuvering speed accounting for both positive and negative G limits. VA for positive G loads is easy to figure in a 2.5 G aircraft, it is:

VA=Vs2.5‾‾‾√VA=Vs2.5

Which is approximately equal to 1.58 times the stall speed. Since VREF = 1.3 VS, one could argue that the best VRA for a 2.5 G airplane is 28% higher than its stall speed.

VA for negative loads on an aircraft with a -1.0 G limit is problematic, because it is equal to the aircraft's stall speed. 14 CFR 25 says this is okay, but it would be a foolhardy speed to fly in turbulence. The Gulfstream 1.5 VS minimum makes more sense.

The display controller gives you an accurate VREF based on the flap handle position all the way to 15,000' and from there all the way to 45,000' it appears to be very close.

See: G450 VREF / Display Controller for more about this.

Your display controller offers a ready source of VREF for all flight conditions, provided the flap handle matches the position of the flaps. Since VREF in this airplane is 30% higher than VS, if you add another 15% you will have a good turbulence penetration speed. Why 15 and not 20? The math:

VREF=(1.3)VsVREF=(1.3)Vs

which means:

Vs=VREF(1.3)Vs=VREF(1.3)

and:

VRA=VREF(1.51.3)=VREF(1.15)VRA=VREF(1.51.3)=VREF(1.15)

Rule of Thumb: Best turbulence penetration speed for a G450 with 0° flaps is its VREF plus 15 percent.

Of course once you put extend the flaps beyond 0 you become a 2.0 G airplane and VA gets a little slower. (41% above stall instead of 58%.) If it is really turbulent once your flaps are extended, a little extra speed may be harmful to the airplane but might be necessary to keep flying.

 References:

14 CFR 25, Title 14: Aeronautics and Space, Federal Aviation Administration, Department of Transportation

Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013.