AP, I understand that the industry world wide switched from four engines to three as an efficiency measure, then switched down to two engines when the FAA amended the rules to allow it to happen.

How many three engine airliners remain in service with major airlines world wide? I am not talking about the outfits with a half dozen aircraft, I mean companies like Champion that had its entire fleet made up of 727 trijets.

Also has anyone looked at the feasability of producing or at least testing monojet airliners? Something like a DC-10 with a single tail engine should in theory be more fuel efficient than a twin jet like the A-320 simply on reduced drag, if for no other reason.

Three engine jets in major airline fleets?

Hmmm, not many at all really. Most of the DC-10/MD-11 series aircraft are now plying cargo routes. A few smaller charter type outfits are still operating the L-1011 and DC-10's. The four engine aircraft have remained as some sort of marketing/safety issue and can be as efficient as the larger two engined aircraft.

The 727 is being phased out rapidly in almost all but a cargo capacity. Fedex still has a bunch of them, but probably not for long. I think some of the issue why you dont see more new three engine designs these days is a complication and safety issue. With an engine in the tail you have some obvious issues which you dont have with all wing mounted powerplants. Soiux City Iowa and United are a case study in that. Their DC-10 shed a fan blade and lost all hydraulics to the tail. With wing mounted pod engines its rare for a failure to cause catastrophic damage to systems.

Personally I would not want to fly a long distance on a single engined aircraft. I doubt the FAA, or ICAO will ever go there for obvious reasons.

Well I would want to see a good track record of safety before I flew on a mono-jet for any long distance, but in terms of efficiency it would offer some pretty significant advantages. I have read that one of the cargo carriers had refitted a number of 727's with new larger wing engines while retaining the tail engine as orriginally installed. I don't remember which one it was so I can't look it up again. How well does a tri-jet fly on a single engine anyhow, is it better or worse than a twin jet with one engine out?

As for wing pods, doesn't a DC-9 use two tail pods instead? Does that pose some sort of added danger in a turbine failure scenario that I am unaware of? Engines now are so much better than they were even twenty years ago that it seems catastrophic failures are exceedingly rare events.

I know almost nothing about planes, but a common sense question that occurs to me is whether the risk of catastrophic engine failure due to objects being sucked in can ever be completely eliminated. If not, then how could a single engine plan ever be viable (unless you sent all the passengers to jump school first)?

In a single rear engine model, it seems that the risk of debris from the plane itself would be higher than for wing mounted engines. Thus, it seems like there would be a design-based increased probability of catastrophic failure in a single engine rear mounted engine over the same engine mounted on the wing.

I am assuming that single rear engine aircraft (like some fighters, I suppose) are not flown nearly as much as commercial aircraft and thus the single engine risk is not as big an issue.

Redundancy is good. No redundancy when 300 lives are at risk is not good.

Thus spake Zardoz.

While it is true that intake of a foreign object can not be 100% avoided and is a major source of turbine failure. When you are talking about a modern turbofan with bypass like is used on most passenger aircraft there are a number of things to keep in mind. One is, the engine intakes tend to be very large as do the blades on the first stage of the compressor. While flying through a flock of birds is never a good idea modern engines are tested before certification to survive injesting large flying birds like geese. The engine needs a complete overhaul and might suffer serious damage, but most of the time can continue to produce power until an emergency landing is made.

When you are talking about ingesting metalic debris, well odds are you are going to have a very bad day when that happens.

For short term flights, say up to 90 miutes, a no power landing is bad but not an automatic high fatality event. Even the Sioux City crash landing AP referred to above was not a 100% fatality situation and that was with no hydraulics and very poor manuverabillity. If you are talking about a damaged engine the crew will be declaring an emergency and landing at the nearest suitible airport. If you are talking about an engine totally out then you are relying on backup systems to provide the hydraulics and you are in a really big glider with a poor ratio. In that situation you are talking about setting down on the first available spot, hopefully something like the Bonneville salt flats but more likely a highway or such where you are praying and hoping all the way to touchdown and through rollout. The Space Shuttle lands that way every trip, but it was designed specifically to do so and has a large group of people planning every landing in great detail. The Monojet would be a whole different kind of glider.

As for single engine military aircraft, planes like the A-6 flew day in and day out for decades before they were retired in the mid 1990's. Military aircraft however have ejection seats and trained crews who know how to use them, that isn't practicle for a passenger jet. The concept is to make the plane lighter and more fuel efficient, with a trade off in the form of less safety.

On the other hand there have been quite a number of single engine civilian craft since the beginning of aviation, they just never caught on for passenger planes.

By the way, there is no reason that a single engine civilian airliner would have to have the engine in the tail, though that is the most common concept. You could also mount it on a fuselage pod like the DC-9 uses forward on the aircraft top, above the wing with a high wing design, or you could mount it in a 'stinger' which would protrude behind the tail where a catastrophic failure would throw debris completely away from the aircraft itself.

http://www.globalaircraft.org/planes/c-5_galaxy.pl

http://www.globalaircraft.org/planes/c- ... rlifter.pl

Imagine that wing shape with a single engine mounted in a pod centered over where the wing and the fuselage meet.
It would be no more vulnerable to debris than a wing mounted pod.

Now picture this KC-10 with just the tail mounted engine and no wing pods...
http://www.globalaircraft.org/planes/kc-10_extender.pl

Or picture a Gulfstream 550 or 650 with a single tail engine and a passenger capacity of 20.

From a strictly mercenary POV, redundency is redundant and expensive.

I am not saying mono-jet airliners are likely to hit the market any time soon, just exploring the feasabillity of a system more fuel efficient than the current twin engine configuration.

New questions for AP

I have been watching a series of lectures on AGW and the section on contrails caught my attention. You are the obvious person to ask about them so here we go.

What is the highest practical cruising altitude for the commercial passenger jets you are rated to fly? Cargo jets too if any are applicable.

Is it true that Contrails rarely form above flight level 390 and are very rare above 450?

If not what level does the formation of contrails start to fall off at and what aircraft can cruise at those levels?

I know that when a jet takes off at max weight it has a hard time reaching a high flight level, but I am given to understand that within two hours enough fuel is consumed to remove this limit. Is that a good rule of thumb or am I misunderstanding something about the physics of flight?

How much extra fuel does a commercial aircraft burn at say FL430 vs FL 350?

Other questions will depend on the answers to these so I will eagerly await your answers.

Generally speaking for those of us who are not firmly ensconced in the "Chemtrails" camp ..contrails are a normal atmospheric phenomenon produced by the combustion of jet fuel and air at high altitudes in the correct temperature/ moisture combinations.

Contrails can form at a wide range of altitudes really and there is no set rule for their generation. Typically we see them above the mid to high twenties at cruising flight levels. They can form up to just about any altitude that a jet engine powered aircraft is capable of operating. Normally most airline traffic is within a band of around 30,000' to around 41000'. Some private jets do cruise higher but that makes up a very small percentage of the total. Military aircraft fall into basically the same categories as airliners when it comes to contrails.

Most airliners I have flown cruise up to flight levels as high as 43000' Again normally the cruise altitudes are within a range of 30-40K. Occasionally you can see contrails at very low altitudes but they are rare below about 20,000 unless you are in arctic regions.

Typically jet engines become more fuel efficient the higher they go. Maximum altitudes are usually a result of the weight of the aircraft and the capability of the airfoil to produce lift in the very thin air of the upper atmosphere. The amount burned to reach a given altitude will depend on things like weight/payload/air temperatures, and aerodynamics. The rule of thumb for jet engined aircraft is that the higher you can get the more efficient you will be.

An aircraft's altitude capability is determined by its aerodynamics(wing), thrust available, and fuel/passenger/cargo load. Two hours is NOT a good rule of thumb to use as far as being able to continue a climb after weight is burned off due to fuel consumption.

Most of the time, when an aircraft reaches the upper levels of the atmosphere (typically at the Tropopause) the air becomes very cold AND dry which will reduce or completely eliminate the formation of contrails, hence you don't see many of them at very high altitudes or above the Trop.

OK so what IS a good rule of thumb, if there is one As I understand it for most commercial aircraft the ceiling is less than theoretical due to full loading so the 2 hour rule was in reference to weight savings, but it seems like that is not a realistic rule.

There really isnt any rule of thumb. Some aircraft can fly up to their design cieling limits at full weights. I'd say that for most aircraft before they travel half the distance of any longer range route, they are capable of climbing to their maximum altitude. Probably a pretty safe bet, but Im basing this on assumption really and my experience.

passenger plane contrail global warming - Google Scholar

Here are some interesting quotes from "Transport and climate change: a review":





Hmmm. Here's a link to that last mentioned paper: Impact of unusually clear weather on United States daily temperature range …

Interesting thread!

I've always assumed that when contrails hang around for a long time, the air above my head is moist, and if the contrails disappear quickly, the air is dry.

I would assume a "flying wing" type aircraft would be more efficient then current designs?

Air travel is going away for most of us, so i doubt much will be spent on new designs. I would think they'll just cram ever more amounts of humans on each flight.