Vx and Vy and commercial a/c

[PantLoad]

Suggest referring to your...

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]

Originally Posted by Escape Path

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.

Originally Posted by Escape Path

...(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.

Originally Posted by Escape Path

...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.

[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]

Originally Posted by galaxy flyer

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 ), 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 .

[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,