Sponsor Message:
Civil Aviation Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Why Twins Are Safer Than Trijets  
User currently offlineCedarjet From United Kingdom, joined May 1999, 8199 posts, RR: 54
Posted (15 years 7 months 1 week 2 days 11 hours ago) and read 1425 times:

I posted this as a response to a topic but is also here under it's own steam. A tri-jet is less powerful and has a lesser max climb performance than a twin. To be certified for operation, a new plane must be able to climb with one engine out. So a 757 must be able to climb with half of it's power lost, whereas an MD11, TriStar or 727 must be able to climb with only a third of it's power lost (not sure of exact rule on 4 engine designs).Therefore, the power in reserve on a twin is correspondingly higher, and a twin at max power would be able to outclimb any trijet, let alone any 4-engine design. This is why in some circles (the author included), twins are safer than 3- and 4-engine designs because most accidents occur on take-off and landing, virtually never over the ocean and never ever because of mechanical problems with engines (since jets). The greater power-to-weight ratio in a twin gives an extra margin of power when it may be most needed.


fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
18 replies: All unread, jump to last
 
User currently offlineBryanG From United States of America, joined May 1999, 436 posts, RR: 0
Reply 1, posted (15 years 7 months 1 week 2 days 11 hours ago) and read 1420 times:

There are some situations where (1) trijets are safer, some where (2) twins are safer, and some where (3) it really doesn't make a difference.

(1) Despite the fact that all planes in the air are safe, if I had to be in a plane that lost power in one engine right after takeoff I'd want ot be in a trijet or a quad. There's a lot more room for error if you're in a plane that loses only 1/3 of its power at takeoff than if you're in a plane that loses 1/2 of its power.

(2) Trijets are always old planes (except MD-11). That means the engines are antiques and it takes them a lot longer to build up power. Trijets also don't have quite as much reserve power. That means if they get hit by windshear a twin with two big new engines will stand a better chance of survivng than a trijet with old, slow engines. That's why you'll never, ever, ever see a 757, 767, or 777 get taken down by windshear.

(3) If an engine fails at cruise out over the ocean, it doesn't matter what kind of plane you're in becasuse the chances of another one failing are almost impossibly small.


User currently offlineFlyf15 From , joined Dec 1969, posts, RR:
Reply 2, posted (15 years 7 months 1 week 2 days 11 hours ago) and read 1419 times:

Also, with trijets, if one engine fails, the plane of thrust is going to be much closer to the center of gravity then with a twin, or in simple terms, its going to be easier to control.

User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 3, posted (15 years 7 months 1 week 2 days 10 hours ago) and read 1421 times:

This statement is baltantly false. Common sense dictates that if you lose an engine on a tri-jet then you have 2/3 available power remaining. On a twin it's 1/2. Secondly on a 3 engine jet if you have a power loss on any of the power plants there is significantly less assymetrical thrust. And to add insult to injury. Anything at max gross weight departing a hot and high altitude airport with a 50% power loss would probably not have a positive rate of climb. A tri jet losing an engine high and hot would have a lot less trouble. I've done it in both. They're both walk a thin line between dying and flying. More so on the twin.

JETPILOT/CIME/FE/DC8/727qualified


User currently offlineCedarjet From United Kingdom, joined May 1999, 8199 posts, RR: 54
Reply 4, posted (15 years 7 months 1 week 2 days 10 hours ago) and read 1423 times:

I posted the original message, and JETPILOT is right in that 3 or 4 engines is better in the event of an engine failure. My point is that mechanical engine failure is a very rare occurence and I know many lifelong pro pilots who have never experienced a shutdown in flight ever. But with all engines turning a twin has more power in reserve and as BrianG pointed out for me, a 757/767/777 is much safer in windshear or other unusual events, especially weather related ones, or in the event of a sudden climb being needed to perform a missed approach or to avoid collision, a much more regular (by comparison) situation and one that nearly all pilots have experienced more than once.


fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 5, posted (15 years 7 months 1 week 2 days 9 hours ago) and read 1419 times:

I see what your trying to say now. Could be. It's worth the thought.

User currently offlineLufthansa From Christmas Island, joined May 1999, 3224 posts, RR: 10
Reply 6, posted (15 years 7 months 1 week 2 days 8 hours ago) and read 1419 times:

there is one thing fundimentally wrong with this arguement.

Cedarjet claims that with all engines running a twin has substaincially more power in reserve than a quad or trijet. BS. Its simple grade 3 maths. Okay if each engine in a twin has 50% excess thrust, and each in a tri has 33.3333% excess and those of a quad have 25% excess, the power in reserve with all engines running is the same! 1 lb of thrust is one pound of thrust regardless of where and how it is sourced. Looking at mere ratios of total power doesn't show the whole and true story. Sure, each engine doesn't individually have as much power in excess, but that is irrelenvent, as, we fly using all of an aircrafts engines and thus it is approprate to sum the total excess thrust (not just looking at one engine). Really, this post makes no sence whatsoever.


User currently offlineHiker From , joined Dec 1969, posts, RR:
Reply 7, posted (15 years 7 months 1 week 2 days 7 hours ago) and read 1419 times:

I just know you can dispatch a Tri-star with one engine out on a ferry flight [Not the #2 either], I wouldn't want to be in a twin [Not that it's allowed] in that situation.

User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 8, posted (15 years 7 months 1 week 2 days 6 hours ago) and read 1419 times:

This whole post is unsubstantiated until someone can post the applicable FAR's regarding rate of climb requirments for aircraft operating with inoperative power plants. It is written in stone somewhere. But I have no idea where to look. Any Ideas. The FAR's I have don't cover aircraft certification. Any help/

User currently offlineMD-11 From , joined Dec 1969, posts, RR:
Reply 9, posted (15 years 7 months 1 week 2 days 5 hours ago) and read 1419 times:

If an MD-11 has more total thrust then a 777, then I don't see how that makes the 777 more safe in for example a windshear situation just because it is a twin jet. If a plane has more power then another plane, it doesn't matter on to how many engines that power is distributed, it will still always have more power... according to my simple thinking.

User currently offlineDeltaAir From United States of America, joined May 1999, 1094 posts, RR: 0
Reply 10, posted (15 years 7 months 1 week 1 day 22 hours ago) and read 1419 times:

The L-1011 came before its time, and was supurior to the other planes of that time. In regards to your comment about older engines being less powerful and taking more time to build up it is true, at least compartively speaking. I know that 20 years ago these engines were top of the line, now come now this plane has made it this far lets give it a break. Delta Airlines's L-1011-500's have been refitted with newer engines that have more thrust and have quicker resopnse, at leaast some of them have. Just because its older doesn't mean it cant handle stress.

User currently offlineIce Cream Man From United Kingdom, joined May 1999, 127 posts, RR: 0
Reply 11, posted (15 years 7 months 1 week 1 day 19 hours ago) and read 1419 times:

Here are some climb gradients in case of a go-around for you:

A-330 - 4.0 percent
A-300 - 3.5 percent
A-320 - 4.0 percent
B-757 - 5.0 percent
DC-10 - 4.5 percent

So what this shows is that twin engined planes do not perform better than three engined ones perse. In fact, even between twin engined planes there is a lot of variation.

As far as a plane being less safe because arguably it has got less power (excess power) available is rather trivial. The power available in normal operations is always plenty with big jets. Compare it to turboprops and you know what I am talking about.

Saying the 777 and it's cousins will never, never ever come down due to windshear seems extremely ignorant. You be surprised at the force of nature.


User currently offlineDC-10 From , joined Dec 1969, posts, RR:
Reply 12, posted (15 years 7 months 1 week 1 day 18 hours ago) and read 1419 times:

If i were in trouble i know i'd feel much safer on a tri or quad engined jet. As for twins having more power i don't know that i agree with that. The engines on an MD-11 have in excess of 60,000lb thrust per engine i believe. As well there are no ETOPS on trijets. All aircraft were designed with efficiency in mind first....The fact that a plane is older doesn't make it unsafe. If DC-10s and L1011's weren't air worthy they would have been pulled out of the sky years ago. However it was demonstrated that a twin engine can make it safely across the Atlantic so airlines figured why do with 3 engines what you can do with 2. So a big move to twins was made.

User currently offlineJZ From United States of America, joined May 1999, 252 posts, RR: 0
Reply 13, posted (15 years 7 months 1 week 1 day 16 hours ago) and read 1419 times:

I don't agree with your opinion.

First of all, just from a probability point of view, it's a more likely to have a simutaneous dual-engine flame out than a tri- or quad-engine flame out, during any phase of the flight. A twin with dual-engine flame out is doomed during take off; while a tri-jet losing 2 engines still has a chance, and a quad can more than likely return safely.

Second, reserved power and climb-rate is meaningless in this comparison. I don't know where you got your information on twin can always climb faster than tri-jet. The reserve power is available to engines on tri- or quad-jets as well. And so what a twin can climb a little faster than a tri- or quad-jet. You are not running away from AAA or SAM.

Just from a psychological point of view, a twin pilot facing a one-engine flame out will be under more pressure than his counterpart in a tri- or quad-jet faceing the same situation.


User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 14, posted (15 years 7 months 1 week 1 day 9 hours ago) and read 1419 times:

How can you post the climb gradient for airplanes when you don't know all the factors involved in calculating it. The first and foremost is takeoff weight followed by density altitude.

And climb gradient is expressed in feet gained in distance flown. Percentages hold no value in this context.


User currently offlineIce Cream Man From United Kingdom, joined May 1999, 127 posts, RR: 0
Reply 15, posted (15 years 7 months 1 week 1 day ago) and read 1419 times:

First of all, feet gained in distance flown is can be expressed in a percentage and that is what was done here. Just because in your neck of the woods things are expressed a certain way doesn't mean that that is the case anywhere.

Secondly, the climb gradients that I mentioned are for the max.landing mass (remember it was missed approach, not take-off) of those particular planes, pressure altitude 3000 ft and ISA+15C. What the density altitude is doesn't matter in this context since they will all be affected by it equally.


User currently offlineCaptPeter From , joined Dec 1969, posts, RR:
Reply 16, posted (15 years 7 months 6 days 17 hours ago) and read 1421 times:

If a plane loses an engine it does not have to have a certain amount of power as long as the planes abaility to stay airborne, above a certain height and be able to land at an airport that is all that matters. At YYZ, one of the guys in the control tower told me that a Tri-jet would be easier to operate with an engine down then a twin-jet as it stay balanced which in turn uses less power so it is just as safe. In 1995, NASA engineers used an MD-11 to do the first ever no-engine landing proving that is is possible for even no engines that a plane could land, but an MD-11 could also stay aloft with only 1 engine operating as long as you don't go too fast or something like that.

User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 17, posted (15 years 7 months 6 days 14 hours ago) and read 1420 times:

That test that you are refering to was not a no engine landing. It was a no flight control landing to see if a new flight control system using only the engines power would be able to land an airplane. It was as you pointed out successful. Thus was prompted by the United Airlines Siox City, Iowa DC-10 crash.

User currently offlineCedarjet From United Kingdom, joined May 1999, 8199 posts, RR: 54
Reply 18, posted (15 years 7 months 6 days 13 hours ago) and read 1422 times:

Thank god for that. An MD11 dead-stick landing would be a fearsome task considering the aircraft's infamous tendency to pitch up dramatically during the flare (dramatically demonstrated at EWR by a FedEx plane), resulting in a very high incidence of tail-strikes.


fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
Top Of Page
Forum Index

This topic is archived and can not be replied to any more.

Printer friendly format

Similar topics:More similar topics...
Seats Backward Are Safer: So Why Are'nt They Used? posted Tue May 2 2006 21:48:47 by Dsa
Why No 777-100 Than 767-400er posted Tue Sep 12 2006 15:17:11 by Albird87
Why Not Parachutes, Rather Than Life Jackets? posted Fri Jul 21 2006 17:16:43 by EZYman
Why Is AA Cheaper Than BA For The Same Flights? posted Thu Jul 20 2006 22:55:14 by Highpeaklad
You Guys Are Smarter Than You Thought Part I posted Sun Sep 18 2005 21:59:27 by Dougloid
Why Do F/A's Make More Than CSR's? posted Sat Aug 27 2005 00:57:11 by Bridogger6
B777-300: Why 50m$ More Expensive Than B747-400? posted Wed Aug 17 2005 17:13:51 by Vfw614
Which Airplane Are Larger Than The A380? posted Tue Jan 11 2005 08:47:16 by A380900
PHX Terminal 2 Why Is It Different Than The Others posted Mon Jan 3 2005 21:51:55 by Hamad
Why Airlines Are Doing So Poorly posted Fri Nov 5 2004 18:41:55 by Flyabunch