Should You Maintain Vref?

There are dozens and dozens of target airspeeds and their abbreviations all start with the letter “V” for velocity. Every private pilot is required to learn and recite Vx for his airplane, which is the best angle of climb airspeed. And also Vy which is the best rate of climb airspeed. And probably Vso which is stalling airspeed in the landing configuration.

Va is also a biggie in any airplane because that is maneuvering airspeed, the maximum speed at which you should make full or abrupt control inputs. But one of the most important airspeed targets for pilots of jets or larger airplanes, Vref, hardly ever gets a mention in piston airplane flying. I think Vref deserves more attention.

Vref is the final landing approach airspeed target with the airplane configured for landing. The goal is to be stabilized at Vref airspeed and cross the runway threshold at Vref. It is the only airspeed that I can think of where pilots get no slack on the minus side during a checkride. Other target airspeed typically have a tolerance of plus or minus 10 knots, but for Vref, the rules are plus 10 knots is okay, but there is no tolerance for flying even one knot slower than the target.

Vref also comes with maneuvering restrictions. Typically you can slow to Vref only when bank angle is limited to 15 degrees or less.

Vref airspeed is determined during certification flying but is usually 1.3 times the landing configuration stall speed. Of course, stalling speed varies with aircraft weight, so at higher landing weights Vref is faster. Airport elevation, air density, runway length or other considerations on approach do not change Vref, only aircraft weight does. What Vref gives pilots is a 30 percent airspeed margin above the stall.

The concept of Vref is nearly as old as flying itself. Pilots quickly learned that if they slowed too much on approach the airplane could stall and hit short of the runway. Flying slowly in gusty conditions could also change an otherwise safe approach airspeed into an unexpected stall. But carrying too much airspeed on approach leads to floating and a long landing that could send you off the far end of the runway.

It was, however, the introduction of jets with their high drag and heavy weight on landing approach that enshrined Vref as the most important airspeed for all maneuvering in the airport area. Pilots who flew too slowly on approach in a jet could find themselves in a serious sinking spell. Jet engines take several seconds to spin up to full power so without a margin above stalling  speed there was no way to arrest the sink. The 30 percent airspeed margin built into Vref is the cushion a pilot needs to recover from an unexpected sink.

In piston airplanes power response is quick, and the drag of flaps and leading edge devices is not as high as on a jet so Vref has not carried the same emphasis. Pilots of piston airplanes can get away with flying much slower than Vref even in windy and turbulent conditions—most of the time.

But why should piston airplane pilots give away the safety margin maintaining Vref provides? And maintaining Vref target on final brings predictability to every approach. If you always cross the runway threshold at Vref airspeed your touchdown point on the runway will be predictable every time.

To determine Vref airspeed for your approach you need to know what the airplane weighs. I mean what it weighs on approach, not what it weighed for takeoff, or what maximum certified weight is, but what the airplane weighs right now. In most piston singles the difference in stalling speed between the maximum and minimum landing weights is only a few knots, but remember, Vref is a very precise target to fly to.

In my Baron Vref varies from 96 knots at maximum certified weight, to 85 knots at the lowest weight I’m likely to land at. Many airplane flight manuals provide estimates of how much landing distance each knot above Vref adds but mine doesn’t. But, at 90 knots groundspeed, the airplane is covering 150 feet per second. If you float for five seconds 750 feet of runway is behind you so you can see that every knot matters.

In windy and turbulent conditions it often makes sense to land piston airplanes with less than full flaps, but most piston airplane handbooks provide no partial flap stall speed information. For my airplane I can look and see that at a typical landing weight flaps full down stall speed is 70 knots. Flaps up stall at the same weight is 79 knots. I could estimate that approach flap setting would yield a stall airspeed half way between the two numbers, but that would only be a guess. So I choose the flaps up stall to calculate a Vref of 103 knots. If it’s that windy runway length is shortened so the extra speed should not be a problem to get down and stopped.

Yes, you can fly your whole career in piston airplanes without ever formally calculating or flying a Vref airspeed. In piston airplanes close is usually good enough. But knowing, and flying, a true Vref brings precision to your approaches and landings as well as added safety. I like to have a plan and specific target for everything I do in an airplane.

 

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30 Responses to Should You Maintain Vref?

  1. SkyGuy says:

    - V speed numbers are for high speed aircraft.
    - V speed numbers are not needed for slow speed flight.
    - They don’t go fast enough to matter.
    - Simple….if not off the ground by 1/2 the runway length….stop.
    - On final…..just keep the nose pointed down and the wings near level…no stall will accure.

    • Cary Alburn says:

      With all due respects, your premise is wrong, and your last comment (On final…..just keep the nose pointed down and the wings near level…no stall will accure.) is a potential killer. An airplane’s wing can stall at any pitch relative to the horizon and at any airspeed–basic stuff from early in student training. See page 4-2 of the FAA’s Airplane Flying Handbook: “A stall occurs when the smooth airflow over the airplane’s wing is disrupted, and the lift degenerates rapidly. This is caused when the wing exceeds its critical angle of attack. This can occur at any airspeed, at any attitude, at any airspeed.” V speeds are important for any airplane, unless your definition of “high speed aircraft” is anything that exceeds 20 knots. Seriously–your comment is dangerous.

    • Dave says:

      Great work … Try that method at a density altitude of 11,000′ and report back to us all, will ya? We’ll wait to hear from you.

    • Danny says:

      Really, really bad advice. You should consider remaining silent, as impressionable idiots might believe your lie. I’ve stalled more than one airplane with the nose pointing directly at the ground. You are aware that AOA is NOT the same thing as pitch attitude, right?

  2. Cary Alburn says:

    An easy and not all that costly solution to determining Vref (or simply 1.3 Vso for those so inclined) is to install and calibrate an Angle of Attack indicator, and then make the approach speeds according to its indications instead of the airspeed indicator. When I installed one in my 1963 Cessna P172D, my approach speeds became markedly slower at the light weights at which I most frequently fly, yet they are still 1.3 Vso for the then weight, instead of 1.3 Vso for the maximum gross weight, which is what approach speeds are based on in the POH/flight manual. After flying with the AOA indicator for nearly 3 years now, I believe that they should be standard equipment in all airplanes.

    But without an AOA indicator, another method often used by experienced back country pilots is to determine the indicated stall speed while still a couple of thousand feet above the proposed landing site (just slow to the aerodynamic indications of a stall), multiply that by 1.3, and use that for the approach speed. It won’t be exact, but it’ll be close enough and safe.

    Using the proper approach speed and no more does wonders for short field and spot landings.

    • Tim Metzinger says:

      Cary,

      AoA’s are the best thing ever. I’m hopeful that in the future more airplanes will have them integrated into the GarmiDyne glass panels.

      • DEL says:

        You must mean AoA indicators. An AoA indicator is to AoA (Angle of Attack) as an altitude indicator is to altitude and a tape measure is to length. AoAs occur to me all the time, w/o bothering to buy and install the indicator. Am I to be elated for having the best thing ever all the time for free?

        • Benjamin A. Rolfe says:

          Yes, you should be elated – without a positive angle of attack, you won’t fly! That’s another myth that needs kiling – that the shape of the wing is what makes “lift”. It’s nonesense. Angle of Attack is what makes for the pressure differences that create what we call lift (and what on of my professors in grad school called “suck”, since it is the DROP in pressure due to acceleration that really does the trick ;-) . If that “airfoil shape” were what did it, a airplane, a Super Decathalon, or Sean Tucker wouldn’t be able to fly ;-) .

    • Michael Kobb says:

      I was going to post mention of an AOA as well! Thanks for saving me the trouble. :-)

      Does your AOA adjust for different flap positions? I have read that some do and some don’t.

      • Cary Alburn says:

        It doesn’t adjust for different flap settings, but in practice it makes little difference. Mine is the Alpha Systems analog “mechanical” model. When I calibrated it, I adjusted it with flaps up so that the needle was at the red/yellow mark and the airspeed was at the slowest in which I could maintain full control, per the instructions. That results in a stall when the needle is about in the middle of the red segment. Then I flew it with different flap settings, and there was no noticeable difference–there was full control at the red/yellow mark and the stall occurred in about the middle of the red segment. Other airplanes might be different, of course.

      • Charles Lloyd says:

        It depends on the design. Business jet aircraft ususally have flap compensation designed into the AOA System. The Alpha System AOA systmem installed on GA aircraft does not have flap compensation.

        I have installed a Alpha System AoA 7 year ago on my 182 and clibrated the unit for 1.3 Vso flaps up. At flaps 30 the reference is 1.35 Vso and that is close enough. I fly in and out of a 1,800 long sod field that is not flat. My landings are significantly more consistent with the AoA System that includes audio call outs.

  3. Pingback: Have you thought about Vref? | High Altitude Flying Club

  4. Ramiro Silveira says:

    Mac,
    Good article.
    I would take care with the concept that flying above Vref is safe.
    Many pilots take this for true and include a lot of additives due to wind, gust, extra margin to stall, etc.
    I would say that it is acceptable during the approach phase, but at short final the extra speed should be bled and speed should be reduced down to Vref when crossing 50 ft over the runway threshold.
    This is the only way to make sure that the performance predicted in the manual will be accomplished. As you told, every knot matters, specially for low drag/high ground effect airplanes.
    On the other side, flying below Vref may not be that bad in some cases, because manufacturers must demonstrate satisfactory flight characteristics landing at Vref – 5 kias in order to comply with landing performance requirements (23.75 and 25.125).
    I do not know of this being an issue for Part 23 airplanes, but for Part 25 may be, because those big planes use 1-g stall speed as a reference, thus their Vref is 1.23 times stall speed instead of 1.3.
    Your finish brings the most important messages for me:
    1 – flying a specific speed brings precision and safety to approach and landing;
    2 – have a plan and specific target for everything to do in an airplane.
    Thanks,
    Ramiro

  5. Jim Accuntius says:

    Was taught to use VREF of 1.3 VSO and adjust for partial flap or other conditions 40+ years ago. It still works well.
    The number that is more important to me is that the value of my Comanche decreases in inverse proportion to the price of 100LL. Double the price equals half the value.

  6. Tim Metzinger says:

    Vref (1.3 Vs0) is what I was taught and what I teach for the short final approach speed for a “normal” landing. However, for a short field I teach 1.2 Vs0 on short final.

    Here are some numbers for a 172S, in the 30 degree flaps landing configuration.
    Vs0 is 40 KIAS. That’s pretty accurate based on the last time I went up and flew at Minimum Controllable Airspeed. Stall warning horn blaring, airplane buffeting, but not QUITE stalled yet.

    Vref(normal) would then be 52 KIAS
    Vref(shortfield) would be 48 KIAS.

    Cessna’s AFM calls for 65 KIAS on final for a normal landing and 61 KIAS on final for a short field landing – both higher than Vref.

    So what I normally see is 65 KIAS at the start of the final leg, slowing to 55-60 over the numbers. I don’t want to see less than 55 until the landing surface is underneath the wheels, unless I’m flying with a very proficient pilot who I can trust to lower the nose a little and add a lot of power if the airplane gets mushy.

    Interestingly, in the DA40, Vs0 is 49 and final approach is 65, which is a lot closer to the 1.3 Vs0 we’d expect.

    I believe it’s due to the fact that speed will drop off very quickly in a Cessna with an increase in angle of attack, and not so quickly in the Diamond. Mooneys are similar to the Diamonds in that if you’re too fast you WILL float forever, so they have you come down at 1.3 Vs0 in those airplanes.

    • Dan says:

      Tim,
      Remember to teach the manufactor recommended first or in its absence 1.3 Vso. The PTS is quite clear on this.

      Dan
      DPE

  7. Jim Hardin says:

    Mac needs some practice with Full Flaps and gusty weather! Next thing he will be telling us is that Downwind Turns are dangerious…

    ALL Approaches should be at a “Vref”, in all aircraft!

    It need not be a coldly calculated Vref but a fixed number should be used, never the less or an accepted slop attitude creeps in.

    This only lead to decaying proficiency.
    (bet you never expected that flap comment to draw such attention)

  8. Richard Smith, ATP, CFII, MEI says:

    As a CFI for 43 yrs and flyer to coming up on 50 yrs, plus 3 decades running an aircraft business, typed in six jets, I found this quite interesting. Basically good article.
    Skyguy, one of the first commenters needs some serious flying lessons and lots of ground school if he is to continue flying. I agree with Jim Hardin when he says Mac needs lessons landing full flaps in high crosswinds. I have landed full flaps in EVERY aircraft I have ever flown since I’ve been flying (24,000+ hrs worth) and have never had a problem. But I’m also an OLD stick and rudder guy, not many of us left.
    Any rate Mac, good article, good topic. Some very good responses too, I might add.
    Most informative.
    Richard Smith, ATP, CFII, MEI

  9. Craig Maiman says:

    Hi,

    Remember that the 1.3 Vso calculations for Vref should use CAS, not IAS. So if you’re looking for precision, you need to know the Vso in CAS, do the multiply and then convert back to IAS. It’s not a huge difference, but if you want to be precise, that’s the way to calculate it.

    Craig

  10. DEL says:

    An interesting article leading to important discussions. Thank you, Mac.

    On my STOL ICP Savannah (aerodynamically similar to the Zenith CH-750,) with the help of my instructor, I used trial-and-error to establish the best short-final IAS as 55 mph. This value is a far cry from the nominal 1.3Vso = 39 mph and even 1.3Vs = 45.5 mph. If I begin the flare much slower than 55 mph, an excessive sink rate ensues which causes an ugly thump. If I begin it at a much faster speed I can expect long balooning.

    As to full flaps for landing, there’s no way I can routinely do that. The Savannah has mechanically operated full-span flaperons and I just don’t have enough muscle force to deflect the flaps to their full 36-deg. position at a speed higher than 40 mph. This option I reserve for emergencies only. (I’ve never yet met the runway that really asks for it.)

  11. Thomas Boyle says:

    I’m going to jump in and defend Skyguy a little, here.

    First, on whether “V speeds” are “important” for low-speed aircraft. It’s easy to forget that intuitions and lessons learned in one type of aircraft may be completely wrong in another. For slow airplanes, approach speeds should routinely be much higher than 1.3x Vso (for example). Why? Because ordinary everyday winds have shear that can approach or exceed 0.3 x Vso, while, for a slow-flying airplane, the typical American paved runway is effectively infinitely long. For a slow airplane the risk of overshooting on most runways is likely minimal even at speeds of 1.5x Vso, while the risk of stalling – or at least arriving on the ground with a very high sink rate – is substantial in normal conditions at 1.3x Vso. While jet jockeys, and pilots of turboprops, twins, and even Bonanzas and Cirruses may need to really limit the speed on approach, most of the time the pilot of a slow-flying aircraft will want to “drive it in”, all the way to the flare. Of course, many jet jockeys, etc., who pride themselves on their advanced skills, will try to fly slow airplanes this way, and the LSA statistics bear witness to the results: they bend more LSAs than student pilots do.

    Second, on the comment that if the nose is below the horizon, you’re not going to stall. Sure, it’s true that you can stall an airplane in any attitude, but in actual practice, if you’re on a wings-level approach in a nose-low attitude, you’re not likely to stall inadvertently. I picked up this point from Tom Knauff, the noted sailplane instructor, who has complained about the FAA’s publications on this point, for this specific reason. Yes, in an aerobatic airplane with a lot of elevator authority you may be able to stall quickly from a nose-low wings-level approach configuration – but not without bringing the nose above the horizon! The approach path is usually about 6 degrees, to as much as 10 degrees, with the nose several degrees higher. Stalling AOA is typically 15 degrees or so. To stall the airplane, you have to jerk the nose up to at least 9 degrees above the horizon. Alternatively, you could try to “mush” it to slow down until it stalls, but again you’ll find it won’t slow down if the nose is below the horizon. I can’t promise it’s absolutely true for every airplane, but Knauff has noted that it’s true for every glider he’s tried (he also flies plenty of power).

    So maybe Skyguy is neither as dangerous, nor as in serious need of flying lessons, as folks apparently think…

    • DEL says:

      Stall may easily happen even when the pilot keeps the nose pointing down, if the airspeed drops enough and the sink rate increases accordingly.

      There are two things to consider: First, there’s a difference between the aircraft aerodynamic pitch angle (that of the zero-lift line) and the pitch attitude the pilot perceives. For example, a representative aircraft in landing configuration* at 1.3 Vso would have an AoA of about 7 deg. If the nominal glide path angle is -3 deg the pitch angle is +4 deg. Thus the pilot may see the nose pointing down, yet the aircraft zero-lift line actually points 4 deg above the horizon.

      Second, stall occurs when the AoA is at its stall value, regardless of the pitch or path angles viewed separately. The AoA, however, equals the difference between these angles and can thus reach its stall value as a result of a high sink rate even if the pitch is held constant.

      Returning to the previous example, if, for any reason, the airspeed drops by 0.3 Vso while the pilot keeps aiming at the same point, the AoA would reach its stall value of 18 deg. Since the pitch stays the same, it’s the path that must take the 18-7=11 deg difference. The aircraft thus sinks 11 deg below the nominal glide path, at -14 deg instead of at -3 deg. Note that if Vso is, say, 40 mph, the implied change in descent rate is from 240 to 880 fpm. But the pilot still sees, and sees to it, that the nose points downward.

      ——————————————————————————————–
      * Flaps fully extended, C_L_max = 2.0 @ AoA = 18 deg, C_L_o = 0.6, C_L_alpha = 4.8.

      • Thomas Boyle says:

        Quite right – which is why you don’t want to be operating an aircraft at 1.3 x Vso near the ground, if 0.3 x Vso is comparable to typical gust strengths – which it is, for slow-flying airplanes.

        That said, slow-flying airplanes don’t need to lose much altitude to recover 0.1 – 0.3 x Vso either.

        I wouldn’t press Knauff’s observation. Obviously, an airplane dropped with zero forward speed and the nose below the horizon, is stalled. I understood his point as meaning “as a practical matter”, comparable to “if you fly your approach at 1.3 x Vso you’re not going to stall”… There are exceptions, clearly. But, my thought was that Skyguy wasn’t being quite as foolish as folks seemed to think.

  12. Borneo Pilot says:

    “Of course, stalling speed varies with aircraft weight, so at higher landing weights Vref is faster. Airport elevation, air density, runway length or other considerations on approach do not change Vref, only aircraft weight does.”

    CG also changes stalling speed, with a more rearward CG giving a lower stalling speed. Which is why for the short airstrips I fly into, if I only have two passengers, I’ll put them in the very back of the 206. It also makes it much easier to flare. ;-)

    • DEL says:

      I can imagine why CG location might affect stall behavior and recovery characteristics, but I wonder how it could possibly affect stall onset speed.

      • Borneo Pilot says:

        Well, for the same reason a higher weight increases stalling speed: Why does increased weight give a higher stalling speed? Because the plane has to generate more lift, and thus fly at an angle of attack closer to the critical angle of attack. For CG, the more forward the it is, the further the weight vector is from the lift vector, and thus the more downward force the tail has to generate to counter that moment. This gives more stability, but the negative effect is of course that the more downward force the tail generates, the more lift has to be generated as well, making the plane fly at a higher angle of attack. This is also why racers go for the most rearward CG possible: less force generated by the tail means less lift needed, less lift less drag, faster airspeed. The most rearward CG within design limits is the most efficient place to fly.

        • Thomas Boyle says:

          It’s actually possible to have an upload on the tail at low speeds – sailplanes do, for example, and canards always do. But the point remains correct, that efficiency rises and stall speed falls as CG moves aft, until you reach the point of unacceptable (in)stability.

  13. DEL says:

    Mac says: “Airport elevation, air density, … do not change Vref, only aircraft weight does.” Strictly speaking, if Vref remains 1.3Vso, this is true only for IAS or CAS, which already include the density effect by virtue of the airspeed indicator working principle: CAS=TAS*sqrt(density). For TAS, however, we have

    Vref = 1.3Vstall = 1.3*sqrt(2*weight/density/area/C_L_stall)

    which evidently depends on air density and thereby on airport elevation as well.

  14. Benjamin A. Rolfe says:

    A lot of people are using “Vref” as if it is always equivalent to 1.3*Vso. “Vref” means “reference speed”. It is just that – a reference. Using 1.3*Vso AS Vref often works pretty well. For some airplanes you use a different value. For small airplanes, you will find the recommended final approach speed which you should use as “Vref”.
    You don’t need to call it “Vref” to fly a precise approach. That takes attention to details, and practice.

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