Why over two identical engines?

Boeing were invited to tour the Hawker Sidley factory and saw a tri-jet for the first time. They said at the time they liked the idea. British Marketing expertise in those days was dire. Early Rolls Royce engines were also given to some countries as samples for them to copy.

Like the FARs turned out to say, Vmcg must be established with “maximum available takeoff power on the operating engines”. So in case of paper derate, either Vmcg is unchanged or else the full thrust is made unavailable even in emergency.

That is incorrect. The certification says Vmcg is based on max thrust available (ie pushing the levers full forward), not based on the calculated reduced thrust setting. In a reduced thrust takeoff, maximum thrust is always available and that is what Vmcg is based on.

Back to the original intent of this thread, why would a manufacturer/operate limit the capability of their aircraft by eliminating the ability to increase thrust to account for variations in weight, runway length, terrain clearance, temperature, runway elevation, etc.? I can say from experience that these are all things that have lead to needing to use full TO power rather than reduced thrust. Your original statement would have led to us removing gas, passengers or payload because you believe manufacturers should install smaller engines because that is all the thrust we use in 80% of takeoffs. Being in the business of moving people and payload, airlines want maximize those even when conditions change (weight, runway length, terrain clearance, temperature, etc.).

Like the FARs turned out to say, Vmcg must be established with “maximum available takeoff power on the operating engines”. So in case of paper derate, either Vmcg is unchanged or else the full thrust is made unavailable even in emergency.

It is more than a paper-derate of the engine. On modern engines the FADEC is limiting the engine to a lower output, so even if the pilot wanted more they will not get any more than that allowed by the FADEC, even if the component of the engine allow more.

If I recall, Boeing just waited and watched what the final result was and made the 727 to match. Is this correct?
I suppose someone on the Historic Aviation forum would know.

The Trident had the extra engine because of BEA continually changing the spec.This screwed up the design and caused many delays for the first tri-jet.

Derated takeoffs change V1, VR, V2, second segment climbs, etc. Should Vmcg be calculated it would change as well.

Like the FARs turned out to say, Vmcg must be established with “maximum available takeoff power on the operating engines”. So in case of paper derate, either Vmcg is unchanged or else the full thrust is made unavailable even in emergency.

Looking for the FAR definition/certification criteria for Vmcg I came to this.

(f) At the option of the applicant, to comply with the requirements of §23.51(c)(1), VMCG may be determined. VMCG is the minimum control speed on the ground, and is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), as limited by 150 pounds of force, and using the lateral control to the extent of keeping the wings level to enable the takeoff to be safely continued. In the determination of VMCG, assuming that the path of the airplane accelerating with all engines operating is along the centerline of the runway, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centerline is completed may not deviate more than 30 feet laterally from the centerline at any point. VMCG must be established with —

(1) The airplane in each takeoff configuration or, at the option of the applicant, in the most critical takeoff configuration;

(2) Maximum available takeoff power on the operating engines;

(3) The most unfavorable center of gravity;

(4) The airplane trimmed for takeoff; and

(5) The most unfavorable weight in the range of takeoff weights.

And from the charts I’ve referenced it is based on temperature and weight.

So, suppose that a takeoff thrust is selected, lower than the available emergency thrust, Vmcg is lowered based on the selected takeoff thrust and V1 is lowered accordingly, so that it is bigger than Vmcg for selected takeoff thrust, but lower than what Vmcg had been for available emergency thrust.

And right after V1 – still below what Vmcg had been for available emergency thrust – one engine fails.

What thrust will then be applied by the remaining engine – the selected takeoff thrust, or the available emergency thrust?

None of your situation matters as Vmcg is based on full power being used in the operating engine(s).

So what’s your point again? You want smaller engines installed on all airplanes because reduced thrust takeoffs are an indication that those big fancy engines aren’t really needed? This is one of the most bizarre aviation ideas ever.

Vmcg is estimated base on weight and selected takeoff thrust. The definition of Vmcg is the minimum speed an aircraft can maintain directional control on the runway using rudder only at takeoff thrust. On a derated takeoff all v-speeds are calculated based on that selected takeoff thrust, the current weight of the aircraft and pressure altitude, that includes Vmcg if an aircraft has one. For a given weight the value of all v-speeds will vary depending on the selected takeoff thrust setting and temp/altitude.

The use of the remaining available thrust after an engine loss has no impact on Vmcg. Vmcg can’t exceed V1 and any emergency situation prior to V1 requires aborting the takeoff and not flying. In other words, once the takeoff is aborted that excessive thrust will never be applied no matter what takeoff thrust setting that was selected.

So, suppose that a takeoff thrust is selected, lower than the available emergency thrust, Vmcg is lowered based on the selected takeoff thrust and V1 is lowered accordingly, so that it is bigger than Vmcg for selected takeoff thrust, but lower than what Vmcg had been for available emergency thrust.

And right after V1 – still below what Vmcg had been for available emergency thrust – one engine fails.

What thrust will then be applied by the remaining engine – the selected takeoff thrust, or the available emergency thrust?

Vmcg is estimated for the case of one engine out and the other providing full thrust near V1. Which is an emergency situation to begin with. So how can Vmcg be changed for derate if the derated thrust would be exceeded in emergency (and take the airplane right off the side of runway at high speed)?

Vmcg is estimated base on weight and selected takeoff thrust. The definition of Vmcg is the minimum speed an aircraft can maintain directional control on the runway using rudder only at takeoff thrust. On a derated takeoff all v-speeds are calculated based on that selected takeoff thrust, the current weight of the aircraft and pressure altitude, that includes Vmcg if an aircraft has one. For a given weight the value of all v-speeds will vary depending on the selected takeoff thrust setting and temp/altitude.

The use of the remaining available thrust after an engine loss has no impact on Vmcg. Vmcg can’t exceed V1 and any emergency situation prior to V1 requires aborting the takeoff and not flying. In other words, once the takeoff is aborted that excessive thrust will never be applied no matter what takeoff thrust setting that was selected.

That excess thrust is available to allow variations in takeoff weights, runway lengths, runway slope, terrain, etc. You’re suggesting that manufacturers eliminate that excessive power rather than permit derated takeoffs or they put all their eggs in 1 or 2 baskets by having 1 or 2 engines provide a majority of the thrust with others providing less? Operators then can kiss goodbye the cost savings of being able to derate a takeoff should it be needed or enjoying increased payload or operating at high altitude airports or terrain intensive regions, etc.

I’m not knowledgeable in this field, I had heard though that the two outer engines on the A380 had no reverse thrust. Maybe the facility is there, just not available ?

Derated takeoffs change V1, VR, V2, second segment climbs, etc. Should Vmcg be calculated it would change as well. The same could be said for Min vs. Max weight takeoffs. I don’t know what you’re implying with your question.

Vmcg is estimated for the case of one engine out and the other providing full thrust near V1. Which is an emergency situation to begin with. So how can Vmcg be changed for derate if the derated thrust would be exceeded in emergency (and take the airplane right off the side of runway at high speed)?

Oh, hold on. Wasn’t there one with a small rocket engine in the tail for assistance with take-offs?

Lets also increase the maintenance costs for the aircraft!

Incidentally, can anyone name any aircraft which did have non matching engines (other than engine test aircraft like RR’s 747, and APU’s)?

I can name one, the Trident 3, which had three speys, and an extra RB162 in the tail!

I’m trying to think, but I suspect that the Trident 3 was the only example.

Derated takeoffs change V1, VR, V2, second segment climbs, etc. Should Vmcg be calculated it would change as well. The same could be said for Min vs. Max weight takeoffs. I don’t know what you’re implying with your question.

You’ve answered your own question and probably don’t realize it. Derated takeoffs extend the life of the engine but don’t change the amount of available thrust in an emergency situation.

What exactly do paper derates do to the Vmcg?

But not worse than losing one of the two engines on a twin.

Right, so if it’s not worse than a scenario on twin engine than why is it a concern on an aircraft with more engines? By your suggestion, having 1 or 2 particular engines with a higher thrust rating is a recipe for worse scenario should they be the ones that fail.

It would be no different that having 1 of 4 backup computers be responsible for 80% of the aircraft systems. There would be a 75% chance that the other, less critical, computers would fail increasing your odds of having less of a performance impact when one of them fail. Still, there is a 25% chance that the computer responsible for most of your systems would fail leaving you in a worse position than if the load had been shared equally across all 4 computers.

A lot of planes do add software to flight computers, and have paper derate of MTOW and engine thrust without actual physical alterations of structure or engines.

You’ve answered your own question and probably don’t realize it. Derated takeoffs extend the life of the engine but don’t change the amount of available thrust in an emergency situation. By your original post you’d rather just seen smaller engines. Well that would give you the same effect of a derated takeoff 100% of the time but it would also permanently lower your amount of available thrust should you need it in an emergency (ie An engine loss).

Engines derated on paper have the same size, weight and drag as engines providing their full rated thrust. So why do lower MTOW variants of airliners not have physically smaller engines installed?

Engine size (or lack of) isn’t an indication of engine differences. There could be different fuel flow controllers, ignition systems, defusers, pumps, mounts, etc. installed. Look at the variation of performance on the CFM series for the 737NG.

http://en.wikipedia.org/wiki/CFM_International_CFM56#CFM56-7_series

Because it all fits into the same nacelle the amount of parts needing to be stocked is significantly for those carriers who operate more than 1 variation. Not to mention that there would be less engineering involved with engines that are the same size than modifying an airframe to handle the different aerodynamics resulting from having physically different engines installed on different aircraft versions.

The Antonov 24/26 and maybe the 32 use the APU, which is mounted behind the right engine, for extra thrust at takeoff. According to the crew I spoke to, this is a published procedure and is in the flight manual.

An-26 24.9K Curl – Tactical transport, development of the An-24. Twin-engined short to medium-range military and commercial freighter. Two 2820 shp Ivchenko AI-24T turboprops. The An-26 like the AN-24RV has an auxiliary turbojet in the right engine nacelle, used for high-and-hot operations. The An-26 has a rear loading ramp. 1969.

Reference

Why would a manufacturer design to have different thrust on different engines? Sure losing the less powerful engine would be less of an issue but on the flip side losing the more powerful engine would make a bad situation evenworse.

But not worse than losing one of the two engines on a twin.

I wouldn’t say a major constraint is asymmetrical thrust from an abnormal situation. It’s a lot easier to fiddle with the size of the rudder and add new software to the flight computer than messing around with engines.

A lot of planes do add software to flight computers, and have paper derate of MTOW and engine thrust without actual physical alterations of structure or engines.

lets have all this extra drag from something which isnt doing much…

Engines derated on paper have the same size, weight and drag as engines providing their full rated thrust. So why do lower MTOW variants of airliners not have physically smaller engines installed?

Lets also increase the maintenance costs for the aircraft!

Incidentally, can anyone name any aircraft which did have non matching engines (other than engine test aircraft like RR’s 747, and APU’s)?

I can name one, the Trident 3, which had three speys, and an extra RB162 in the tail!

Sure why not… lets have all this extra drag from something which isnt doing much… :rolleyes: