Why don't Transport Category aircraft have a listed Vy?

Question

I was talking with a pilot about an inability to maintain the targeted climb rate in a particular situation and asked him what the Vy was for the jet. He curtly replied "Vy is only for Part 23 aircraft." (FYI for those outside the US: Part 25 aircraft are commerical planes, while Part 23 are usually general aviation planes. You can find more details here.)

Yet, I know for a fact that there is a maximum excess thrust speed, because at a certain speed drag is at a minimum. This should be close to Vy. I also know that rate of climb is frequently listed for these aircraft. However, the optimal speed for gaining altitude isn't usually listed for commercial aircraft like it is for GA aircraft. In fact, many of the v-speeds like Vmca and Vdec are harder to find for larger aircraft.

Why is this? I can think of several possibilities:

• Commercial aircraft speeds are defined by ATC and operating procedures, not by performance considerations
• Commercial aircraft have enough thrust that climb performance limits isn't an issue (which I doubt from my experience).
• Commercial aircraft use other parameters for similar purposes like rate of climb, v2, or long range cruise speed
• Jets don't list this speed but propeller planes do

Answer 1

Large commercial jets operate within a much wider range of weights than most GA aircraft do, and since Vy varies with weight, it wouldn't make sense to publish "a" single Vy speed for them. The crew can get a best rate or best angle of climb speed from the FMC (which "knows" the current weight of the aircraft, based on current fuel & an entered Zero Fuel Weight).

From an empty aircraft with minimum fuel to a loaded aircraft with max fuel, the weight probably won't quite double for most commercial jet aircraft, but the one might be 170% or more of the other. With that much range, any single Vy you publish would be way far off in a lot of cases. On a GA aircraft, without an FMC and operating in a much narrower weight range, publishing a single speed makes much more sense as the delta between published and actual becomes smaller and the consequence of being a couple knots off is probably slight.

Answer 2

Airplanes certificated under FAR Part 25 (Transport Category Airplanes) must meet far more complex and regulated takeoff performance criteria than can be reduced to a (light aircraft) Vy or Vx (Vyse or Vxse) speed.

If you're taking off in a Cessna 310 and interested in gaining altitude as quickly as possible (rate) then having a published Vy speed (which can be adjusted for weight) is a benefit that the pilot may decide to take advantage of.

On the other hand, in a Transport Category Airplane a particular takeoff path (climb gradient) must be satisfied in accordance with FAR Part 25.111 criteria, which are fundamentally and functionally irrelevant to the establishment of a "Vy" speed (this does not mean a Vy speed for a light aircraft is less important, just less relevant considering the certification regulations).

The issue can be complex, but generally speaking Part 25 airplanes must (depending on the operational regulations involved, e.g., part 121, part 135 ops, etc.) takeoff and climb out using a profile (e.g. speeds, configuration, weight, thrust setting, procedures etc.) that allows for the avoidance of obstacles during the climb. see AC 120-129 "Airport Obstacle Analysis"

For example, during normal Part 121 operations, more often than not, the the thrust setting used for takeoff is less (sometimes far less) than 100%. The amount of thrust used is based on environmental/operational conditions (weight, runway, temp, obstacles, etc.) and computed so that the required climb gradient is achieved.

Performance engineers providing data to the carrier in textual form or programmed data (e.g. FMC) for each airport/runway reduce the very complex amalgamation of regulatory requirements, necessary climb performance, near/far obstacles in the area of consideration, etc. into thrust/airspeed/configuration settings for the crew.

An excellent discussion of this entire issue can be found in this Aircraft Climb Performance NBAA article.

Answer 3

Well they do of sorts. But your pilot friend is right, they are not tabulated for a transport category aircraft like is done for a Part 23 aircraft. Due to the large payloads, CG ranges and configurations, the best rate of climb speed must be computed for each situation. There is no one published Vy for a large transport category airplane.

Incidentally the best rate of climb varies for any aircraft you fly, it’s just that for smaller airplanes that range is quite small. As an example, in a Cessna 172 the best rate of climb Vy varies only a few knots between sea level and its service ceiling. Larger, more powerful GA aircraft will have tables published in the performance and limitations sections of their AFM listing this. Saying that any airplane has a single best rate of climb speed is a bit of a misnomer, similar to saying that an airplane has a ‘stall speed’, etc.

Answer 4

First things first. All the airplanes have a speed for best climb rate (Vy), and this includes transport category aircraft. But the speed Vy, does not matter to us because our climb performance calculations are a little bit more complex when compared to a small GA aircraft.

Think of this. For a general aviation pilot who flies a small aircraft, the main cost he/ she has to bear while flying the aircraft is what he/ she pays for the fuel. Other costs are miniscule when compared to that. So, the aim of a GA pilot will always be to fly while burning the least amount of fuel. Climbing and Vy gives exactly that as it allows for a much more fuel efficient flight, because the faster you reach the cruise altitude the faster you can reduce the power and lean the engine for the least amount of fuel burn. So, Vy matters in a smaller aircraft.

Now think of a transport category aircraft flown by an airline. Yes, the fuel is still a major cost, but an airline also have to pay for the lease or if they own the aircraft they need to cover the capital, they need to pay the pilots, cabin crew and ground engineers and they also need to put the aircraft away for much more longer, scheduled maintenance to keep it airworthy. All of these costs are what we call time related costs. The relationship between these time related costs and fuel related costs is known as the Cost index (CI).

CI = Time related costs / Fuel related costs

As pilots, we insert a CI into our flight management computer. A higher CI means, you will go faster burning more fuel but in the process, you will save time on the trip and save the airline time costs. On the other hand, flying at a lower cost index will burn less fuel and thus saving on fuel costs, but it increases flight time and the result is an increase in time related costs. Based on the CI and very importantly the weight of the aircraft, the computers will calculate an ECON climb speed called an IAS(ECON). And this is the speed that we use to climb at.

If you want the airplane to climb at roughly the best rate of climb, you can set the cost index value to zero (0) in the flight management system. The computers will then calculate an IAS(ECON) to climb, based on the weight. This speed will be the Vy of the aircraft because at a zero cost index the whole aim of the aircraft will be to get to the cruising altitude faster and reduce the time spent in the fuel consuming climb segment so that more fuel is saved. The CI value will vary based on airlines. Some airlines have a very low CI while others have a higher CI. Most airlines prefer to keep it in an in between value, that is keeping a balance between fuel related and time related costs.

Answer 5

Actually modern jets in general do have massive excess thrust. One big reason is due to safety requirements to handle engine loss. Vy is a big issue while on a single engine or piston props. With two turbofans/turboprops running and at low altitudes, they "climb like a rocket" compared with single piston and even twin pistons with big engines.
Additional reasons for the substantial thrust to weight is necessary acceleration for short runway takeoffs and just as importantly to still have excess thrust to climb way up there where the air is thin and high subsonic cruise can be achieved. Up there the airframe and the engine are both more efficient, but due to lost thrust with altitude, you need big powerful engines.
Remember that for jets and high performance turboprops, 3000-4000fpm climbs are common. It just doesn't take long until the terrain is no longer and issue and its more important to get to the destination faster and also save fuel. Flying on Vy typically wastes fuel, because you are running the engine at max continuous power/thrust, while not getting neither the best forward nor the best vertical speeds.
Big jets prefer to climb at speeds above 250KIAS, so they hit the 250KIAS below 10000ft limit.
But strictly speaking, what would be used as Vy, to clear critical terrain right after takeoff, something in the V2+20 to V2+50 range would be used. V2 might actually produce a better angle of climb, but its very uncomfortable to fly that close in case an engine quits. Even then this is used for as short a period as possible. V2+30 requires the aircraft has flaps deployed and is flying at half the speed it would like to go.