V-Speeds for empty planes, Cessna 310Q and Cessna 172

POH & V-speeds

POH v-speeds are typically cited in terms of mtow (max takeoff weight). Reality is, you are rarely flying anywhere near max takeoff weight.

Just how does weight affect v speeds?  We know this formula:
v speed factor = sqrt(weight/mtof)

Real life flying with empty plane

Copy this spreadsheet and modify weights for your own plane. How does this apply to real life, every day flying? If you don't adjust v-speeds for actual weight you are grossly violating vspeeds of a plane. It's alarming just how much. Let's look at two examples.

vRef for a Cessna 310Q is 93kts at mtow of 5500lbs. If you're flying close to empty weight, the adjust vRef is 71kts. You'd be landing 20kts over desired speed!

V-Ref of a Cessna 172

If we look at a very simple plane like a Cessna 172, a vRef of 60kts at mtow becomes 51kts at basic empty. Even if we add a 170lb pilot and 30m of fuel it's a 54kts vRef. You're landing 7 kts over desired speed. Expect to have a very long flare.

VSpeed multiply factors

Below is a chart of vSpeeds for a 310 adjusted between basic empty weight up to mtow.

The vspeed factor at basic empty is just 77% of the published mtow vspeed.

v speed factor = sqrt(weight/mtof)	0.7663	0.8184	0.8674	0.9137	0.9578	1.0000
weight lbs	3,230	3,684	4,138	4,592	5,046	5,500

The full list of 310Q vspeeds are basic empty and mtow are listed here:

weight lbs	3,230
vA (148)	113
vY (122)	93
vG (111)	85
vS0 (72)	55
Vx (80	61
vY (107)	82
vRef (93)	71

vX drops from 80kts to just 61kts. Next time you're doing a short field takeoff, think about this number. You can get more rapid climb performance out of the airplane if you're not full of fuel and passengers.

Hazard with student pilot solo - 30% extra energy on first solo landing!

A friend shared this real world story. He was training with a 240lbs instructor and 50kts approach speed. On his first solo, he had haf fuel, and did the same approach he had been taught. The plane landed way too fast, bounced, bounced again, he went around. Second attempt was also botched. Finally he held it off long enough to land. Let's analyze the difference.

Full fuel and 240lb instructor is close to mtow. Let's call it 2300lbs.
Half fuel without 240lb instructor, let's call it 1952lbs.

50*sqrt(1952/2300) = 46kts. That's the adjusted approach speed he should have used. That's 10% off from what it would have been like with an instructor and fuel. May not seem like much, but let's look at energy relative to stall speeds (when the plane actually stops flying).

vs0 mtow = 41kts.

vs0 at adjusted weight = 41*sqrt(1952/2300) = 37kts

That's a 9kts spread in the normal training scenario, and a 13kts spread in the student pilot scenario. The student was carrying 30% more energy into the flare than accustomed to. This is a big difference!

Takeaways

• Next time you do weight & balance, don't just check off "in envelope" and move onto pre-flight. Make a mental image of your starting and landing weight compared to mtow and how that affects your vspeeds.
• On the runway, think about your adjusted performance speeds. What is your vX? vY? You'll get to altitude faster if you're not speeding.
• During flight: What is your adjusted vG? You'll glide further if you're not speeding.
• End of your trip. Final approach speed?

Quick memory trick for v speed adjust during flight 

We don't have luxury of a spreadsheet during flight, so let's look at the v speed adjust on the 310.

		empty weight adjust
vA	148	-35
vY	122	-29
vG	111	-26
vS0	72	-17
Vx	80	-19
vY	107	-25
vRef	93	-22

We can see that vSpeed drops about 20kts for the close-to-stall vSpeeds, and about 30 for the high-speed v speeds. If you know roughly where your weight profile lies during flight, you can do quick math.

For example if you're flying with 2 passengers and half fuel, you're about 50% to your mtow, so adjust by -10 and -15 accordingly.