Vx and Vy and commercial a/c

[JIC]

Hi

I been wondering quiet a while now about what and how Vx and Vy is used on a commercial a/c.

I am flying ATR and really can't find anything about it in the FCOM's or AFM and the performance books I have been reading tells me that Vx is Vmd on the thrust curve or max excess thrust. Vy is Vmp or max excess power. But how do I find them on my ATR? Do we call them something different on a "big" aeroplane (CS-25)?

Anyone who can give me a hint or maybe have some good links to where I can read more about this?

Thanks

Best Regards Jic.

[Capn Bloggs]

Do we call them something different on a "big" aeroplane
V2...........

[minimumunstick]

V2 is not the equivalent of either Vy or Vx...

[Genghis the Engineer]

They're still called Vx and Vy, but seldom used because transport aircraft work to scheduled performance, rather than best.

G

[hawk37]

JIC, you have it all basically correct, if one assumes thrust is constant (which it may be close to true for an old turbo jet, not that close for a turboprop). Usually the flight manual or performance manual publishes Vy, speed for maximum rate of climb (all engines). Typically though, one climbs at a higher speed, computed to minimize the cost of the flight.
Vx (all engines) is rarely published, and I've never heard anyone use it although I can imagine some scenarios where it would be useful. If the published holding speeds are in fact Vmd, and not some higher speed to aid in aircraft handling, then I'd say this is the Vx speed for that weight.
What one may be able to determine is Vx for one engine inop. Typically, Vfs, the final segment climb speed, is chosen to be the maximum angle climb speed for one engine inop, to allow the best climb gradient.
How do you find Vx and Vy on the ATR? Some elaborate flight tests would get you the answers.
Vx and Vy mean the same for big or small aircraft, regardless of the power plant. Keep googling, and reading.

[Capn Bloggs]

Fair enough. My "V2" was a bit flippant. In the three jets I have flown, none of the documents refers to Vx and Vy. Hawk sounds on the mark re Vx being (close to) the holding speed.

V2 is not the equivalent of either Vy or Vx...
In the engine-out case, what is the difference between Vx and V2? Don't both achieve the best possible gradient for the configuration?

[Wizofoz]

On the 777, the FMC provides the Vx, which is usually around 240-270 kts. Vy is Vref30+ 140 (which is also Vcl + 60).

We use them in the standard way- to meet a lateral requirement (cross X at or above Y) we use Vx, to meet a time requirement (expedite trough X leve) we use Vy.

V2 IS the best speed to fly for obstacle clearence after takeoff, as the acceleration segment to get to Vx would rob to much distance, which I think is what Genghis was alluding to.

[stubby1]

ATR fcom gives best glide speed for obtruction, both engine out as Vmhb0. So this should be the Vx. this is best L/D spd

As Vy is generally more than Vx, Vy will be greater. i guess we'll have to check climb rates at diff speed from fcom.

Another interesting formula is

roc(ft/min) = Pavailable - Preqd/ 33000

hope it helps;)

[Capn Bloggs]

On the 777, the FMC provides the Vx, which is usually around 240-270 kts. Vy is Vref30+ 140 (which is also Vcl + 60).
Are the Vy and Vx values displayed in your FMS or are they implied? Not knowing your exact Vrefs, it seems your "Vx" and "Vy" are almost the same?

[DirectCF]

I could be wrong, but I believe that, on an airbus, Vx = green dot, Vy = climb at cost index 0.

[de facto]

On the 737 NG,the best Rate of climb will be same as ECO CLB at cost index 0....interesting:ooh:

[michelda]

In airbus family.....
Vx=green dot
Vy=climb speed with CI=0.
Michelda

[4dogs]

http://www.pprune.org/tech-log/90019-vx-vs-vy.html#post896930

note the date...:{ :{

Stay Alive,

[Wizofoz]

Bloggsy,

Vx displayed on the CLB page of FMC, headed "Best Angle".

Vy is a "canned" speed, meaning a fixed value over Vref30. No actual figure is displayed, but you get Vref30 from the APP REF page and, if you aren't too tired, add 140kts to it (I usually need the calculator function of my Ipod!!)

[minimumunstick]

In the engine-out case, what is the difference between Vx and V2? Don't both achieve the best possible gradient for the configuration?

In that particular configuration they will probably be very close if not the same. However by definition they are two different speeds and need not be identical. V2 speed is designed to provide maximum obstacle clearance at take-off with one engine out, it will probably not be the speed that will give you maximum angle of climb in the long run, which is Vx (and probably a higher speed than V2 I would think).

I do find it strange how many larger aircraft do not have published Vx and Vy speed, as they are so useful to have knowledge of. With all the other flight tests those aircraft go through I can't imagine it would be difficult to determine Vx or Vy, so why not just add it in the books!?

[sevenstrokeroll]

can you imagine the deck angle/pitch attitude for Vx with all engines running?

15 to 20 degrees would be alot...but imagine you might get close to 23-30 degrees.

anyway...the whole idea might be : you takeoff headed in the direction away from your destination...you want to climb as quickly as possible/angle to get to a place where ATC will allow you to turn towards destination...it doesn't make sense to fly quickly farther away

so you climb at min clean maneuvering

or you leave some flaps out and you climb at min speed for that configuration.


so, nowyou are headed towards your destination and you want to get up high to take advantage of better fuel flow or tail winds...but some ssuper smart guy figured out that the best way to fly is to fly at cruise climb speed as the best compromise for unknowns.

so Vx and Vy are nice to know...but why use them?

[john_tullamarine]

V2 speed is designed to provide maximum obstacle clearance at take-off with one engine out

I don't think so.

The OEM's interest is to sell aircraft and one measure of considerable importance is field length requirement which, in turn, is critically dependent on speed (V2).

The OEM will be looking for the minimum V2 consistent with Design Standards .. ie, driven by Vs or Vmca, although there may be some Type reason to go a bit faster.

Where does this leave us so far as obstacle clearance is concerned ?

(a) for close in obstacles, min V2 will give the best obstacle clearance as you can't go slower and any increased V2 will compromise the clearance.

(b) for distance obstacles, min V2 definitely is NOT the way to go - refer to the use of improved performance/overspeed V2 takeoff schedules.

Figuring Vx and Vy equivalents is straight forward but they are light aircraft considerations, not heavy .. the OEM/operator's ops eng folk will give you information regarding best climb rates.

[minimumunstick]

(a) for close in obstacles, min V2 will give the best obstacle clearance as you can't go slower and any increased V2 will compromise the clearance.

(b) for distance obstacles, min V2 definitely is NOT the way to go - refer to the use of improved performance/overspeed V2 takeoff schedules.

I am confused, doesn't this just confirm what I wrote?

I would appreciate if you cared to elaborate a little on this, I am not sure if I understand:

The OEM's interest is to sell aircraft and one measure of considerable importance is field length requirement which, in turn, is critically dependent on speed (V2).

Thanks

[john_tullamarine]

Probably a touch of wordiology. I am looking at V2 being driven by field length considerations (limited by Design Standards). The reference to field length is to emphasise that. Probably not worth fussing too much over ..

[Escape Path]

In the engine-out case, what is the difference between Vx and V2? Don't both achieve the best possible gradient for the configuration?

There's a slight difference.

Twin Otter figures (at max weight): V2: 80kias. Vx flap 10 (T/O config) is 75kias. So you would climb a tad quicker, but on actual engine failure on takeoff we use V2 and stick the ASI needle at 80. And since the manoeuvre allowances are +/-5kias, the POI doesn't like it when we go below 80kias on engine failure trainings :E

Another thing to point out of interest perhaps, it's that our SOP states that if we are climbing at Vx flap 10 (75kias) we should accelerate to V2+5 (85kias) before flap retraction. The SOP also states that our climb speed on normal circumstances is Vy flap 0 which is 100kias (is there a Vy flap 10?).

By the way, which is the equivalent of, or the closest speed to, best L/D ratio on aircraft like the Twin Otter?

[PantLoad]

Suggest referring to your FCTM or other appropriate publication.

Airbus also publishes Getting to Grips with Aircraft Performance.
This is a great reference manual....will answer most of your performance
questions.

John T is exactly correct, as usual.

In reality, your best angle of climb in the takeoff config...one engine inoperative...is around V2 plus maybe 10 to 15 knots....depending on this and that. (Again, please read Getting to Grips with Aircraft Performance.)
Same situation, but with all engines operating....maybe V2 plus 15 or so.

Best angle of climb, clean, one engine inoperative, is green dot. Same situation with all engines operating, maybe green dot plus a bit (like green dot plus 10 or so).

These figures I cite are approximate. Check authentic manuals/documents for better reference.

For best rate of climb....according to the FCTM, you're looking at around the turbulent air speed (no kidding....it says that in the FCTM). Can't give you a page number....it's a good read.

Someone cited their FCTM (737, I think) which stated to use CI 0.

Interesting...


Fly safe,

PantLoad

[V1... Ooops]

Twin Otter figures (at max weight): V2: 80kias. Vx flap 10 (T/O config) is 75kias. ...on actual engine failure on takeoff we use V2 and stick the ASI needle at 80. ...the POI doesn't like it when we go below 80kias on engine failure trainings...

...we should accelerate to V2+5 (85kias) before flap retraction. The SOP also states that our climb speed on normal circumstances is Vy flap 0 which is 100kias (is there a Vy flap 10?).

By the way, which is the equivalent of, or the closest speed to, best L/D ratio on aircraft like the Twin Otter?

Hello Escape Path:

Although your description suggests you are generally doing things right (right = in accordance with the approved AFM) with your Twin Otter, I am a bit perplexed by some of the figures you have cited above.

For a Series 300 or 400 Twin Otter at MTOW, V2 (Take-off Safety Speed) is 80 KIAS, as you have indicated. During normal two-engine operations, this is the target speed to maintain between 35 feet AGL and 400 feet AGL. Flap retraction is typically commenced at 400 feet AGL. Should an engine fail after V1, the same 80 KIAS is maintained for the entire duration of the climb following take-off, because 80 KIAS is the best rate of climb speed for the Twin Otter wing in the 10° flap configuration, and 10° flap is the only approved take-off configuration for the Twin Otter landplane.

Twin Otter take-off performance calculations are particularly simple because the take-off configuration (flaps 10°) is the same as the configuration for Vyse (also flaps 10°), and the V2 at MTOW (80 KIAS) is identical to the Vyse at MTOW (80 KIAS). Thus, when you make a take-off, you know that your aircraft configuration and speed target will be flaps 10° and 80 KIAS, no matter whether both engine work perfectly or one engine fails after V1.

I am perplexed by your mention of "75 KIAS as Vx flap 10°". I cannot find this specification anywhere in the AFM or the approved AFM supplements. In light of the quite remarkable climb gradient a DHC-6 achieves with both engines operating (approximately 17% at ISA), it seems pointless to even think about a Vx for the take-off configuration. The single-engine take-off climb gradient (again MTOW, ISA) is 4.2%. The only published Vx for the DHC-6 is in the flaps 0° configuration, and at MTOW, that is 87 KIAS.

With respect to flap retraction on the Twin Otter, the AFM states that flap retraction should not be commenced prior to 400 feet AGL (minimum), and that the aircraft should be accelerated from 80 KIAS (V2, also equal to Vy with flaps 10°, also equal to Vyse with flaps 10°) to 100 KIAS (Vy with flaps up and two engines operating) as flaps are retracted.

...(is there a Vy flap 10?)

Certainly. At MTOW, it is 80 KIAS. This is one of the key reasons why 80 KIAS is the V2... because, as mentioned earlier, it is also the Vyse. If something goes wrong on take-off, you don't need to change your flap configuration or your airspeed.

Consider, for a moment, that 'Vy at flap 10°' is 80 KIAS whether you have both engines operating, one engine operating, or even if you are being towed on the end of a rope like a glider. The wing has no idea how many engines are operating, and the speed at which the wing (in a specified configuration) achieves best rate of climb is not going to change no matter how many or how few engines are operating.

...By the way, which is the equivalent of, or the closest speed to, best L/D ratio on aircraft like the Twin Otter?

Glide speed at MTOW (both engines out, both propellers feathered, flaps up) is 100 KIAS for best range and 77 KIAS for best endurance. These figures are published in the AFM for the Series 400 Twin Otter, and will be published in the AFM for the Series 300 Twin Otter when Revision 53 is promulgated.

Michael

[Escape Path]

Hi Michael. Always great to hear from you

I am perplexed by your mention of "75 KIAS as Vx flap 10°". I cannot find this specification anywhere in the AFM or the approved AFM supplements. In light of the quite remarkable climb gradient a DHC-6 achieves with both engines operating (approximately 17% at ISA), it seems pointless to even think about a Vx for the take-off configuration.

In the very short time I've been in contact with the aircraft and those who fly it at my company I've never seen or heard of someone using Vx Flap 10. I've told you previously that we operate with a SOP very similar to the one used by the defunct ACES Colombia (which I've heard was the largest DHC-6 operator at a given point somewhere in the late 80's) so I think you should be familiar with the name and the reputation the airline had. So I am a bit perplexed as well, that there's no reference of that speed other than our company's AFM. I'll scan and send you the page on the AFM that contains the speeds.

Here's what our AFM says about the use of Vx flap 10:

If a Vx-10° flaps climb is necessary after takeoff, corresponding speed should only be set at or above 500ft AGL; accelerate to V2+5 before flap retraction. It is also possible to climb at Vx-0° flaps at pilot's discretion.

With respect to flap retraction on the Twin Otter, the AFM states that flap retraction should not be commenced prior to 400 feet AGL (minimum), and that the aircraft should be accelerated from 80 KIAS (V2, also equal to Vy with flaps 10°, also equal to Vyse with flaps 10°) to 100 KIAS (Vy with flaps up and two engines operating) as flaps are retracted.

Company SOP states flap retraction should be done at or above 1000ft. I think such a high figure is used due to the high elevation/terrain we operate in/around.

Glide speed at MTOW (both engines out, both propellers feathered, flaps up) is 100 KIAS for best range and 77 KIAS for best endurance. These figures are published in the AFM for the Series 400 Twin Otter, and will be published in the AFM for the Series 300 Twin Otter when Revision 53 is promulgated.

Thank you very much.

Best regards

[de facto]

Hi pantload,

in my post (#11),i wrote that best rate of climb vy,(fmc climb page)was the same speed as selecting CI and using ECON climb.

I checked on boeing website and indeed the best rate of climb to save fuel can be found by using a CI of Zero.:8

[galaxy flyer]

V1...Oops

I thought the DHC-6 was a FAR 23 plane. Is it cert'd to FAR 25 performance standard?

GF

[V1... Ooops]

I thought the DHC-6 was a FAR 23 plane. Is it cert'd to FAR 25 performance standard?

Several different certification criteria have been used since DHC-6 production began in the mid 1960s.

The aircraft was originally certified to CAR Part 3. Later, the aircraft was certified to SFAR 23. More recently (2010), an AFM supplement has been published that provides performance information to permit operation to Part 23 Commuter Category requirements (FAR 23 at amendment 58). New production Series 400 aircraft are certified to FAR 23 at various amendment levels.

All of the above information can be found in the AFM. The certification basis is published on approval page 1 of the AFM. Performance data to support SFAR 23 certification is published in Supplement 11 of the Series 300 AFM. The Series 400 aircraft uses SFAR 23 performance data as the baseline.

Supplement 37 to both the Series 300 and Series 400 AFM provides the performance data required to comply with FAR 23 at amendment 58. In practice, this means the data required to comply with EU-OPS Performance A criteria, and to comply with the Canadian and American '2010 rule' requirements for Part 23 aircraft with more than 9 seats.

Michael

[Pontius]

Similar to Wizard's post, the 767's best angle (Vx) can be found on the Climb page and, funny old thing, it comes out as Vref30 + 80 i.e. min clean. Best rate (Vy) is either Vx + 50 or Vref30 + 130 (stunningly close to the 777 :8), whichever you can calculate easier on your iPhone.

There, that's my technical knowledge displayed for the year, so now back to Down Under and hearing about how Australian cadets are going to ruin the World :rolleyes:.

[V1... Ooops]

Guys, Girls:

I think we have to be very careful about accidentally comparing apples to oranges here in this discussion.

The performance rules, certification basis, operational practices, etc. between Part 23 aircraft (generally the 12,500 pound and under crowd, although there are a few derivatives of smaller planes that are heavier than 12,500 in that group) and Part 25 aircraft (Boeing, Airbus, etc.) differ greatly.

Michael

[Che Guevara]

On the A320,

MAX CLB GRADIENT SPEED = G.DOT varies from 205 kt at 60 t to 225 kt at 70 t ➔ + 2 kt/1 t

MAX V/S CLIMB SPD with ALL ENG OPERATIVE ≅ CLOSE TO TURB SPD varies from 279 kt at FL100 to 260 kt at FL250 ➔ - 1.3 kt/1000 ft

That is as close as you get to Vx and Vy etc.

[FE Hoppy]

On the E-jets it's Vfs for Vx and Vfs+50/.6M for Vy.

[galahad]

For ATR
Vx = VmLB
Vy = VmHB

The explanation is in 2.02.01 P3, FCOM Vol 2

Best regards,