It came from Black Background: Matt Shobe's Blog

The basics
Disclaimer: This knowledge generally covers aircraft in service over the US 'cause that's where i've spent 95% of my flight time.

1. The venerable 727

Where have the 727s gone? We always flew those to Disneyworld when I was a kid. There are essentially no 727s (three engines in the rear: two on either side, one above in the tail) in frontline carrier service. They're all too damn old. This is a good thing, trust me, because some of those dogs probably had a few million hours on the airframe (including, possibly, the Trump plane I've seen parked at Laguardia this December) before they went to the boneyard. FedEx still flies cargo in them, but you deserve far better shipping insurance for your bodily self. Still, the 727 was at one time a ubiquitous commercial airliner, in service with just about every carrier worth mentioning.

Today, if traveling in the US, you're far more likely to see American Airlines' MD-80s and Fokker 100s plying the taxiways around the terminal. Northwest operates a fleet of DC-9s, which are for all practical purposes indistinguishable from MD-80s; they are just different series of the same basic rear twin-engine aircraft.

2. Tail-engined peers

MD-80 as flown by American.

This Fokker 100 is difficult to distinguish from the MD-80, isn't it? If you're really into this, you'll note the speed brake retractor arm nacelles (those two 'bumps') on the tail cone — the F100 has 'em, the MD-80 does not.

Today, a large number of aircraft preserve this general silhouette -- cigarette-thin body, T-shaped tail and powerplants in the rear -- because it remains an aerodynamically efficient and cost-effective design. Don't confuse an MD-80, Fokker 70/100, or even a CRJ with the venerable 727 because of it, though. The sure tipoff is that third engine in the tail. There have only been three major American aircraft to integrate a primary engine in the vertical stablizer this way; the other two will get mention later on in this posting. So, unless you're on a charter to Cuba or standing in the Age of Flight exhibit at the Museum of Science and Industry in Chicago, that's not a 727 you're looking out of.

Ok, I get it with the 727. But what about the 737? I can never tell those from Airbuses. Unless you fly a carrier that makes a big deal about its homogenized fleet (JetBlue's all Airbus A320s, Southwest's purely 737s), you are likely to get on a 737 or A319/320 if the flight's less than 1500 miles. As long as the interiors are up-to-date, it'll be hard to tell them apart, inside or out. Here are some keys to look for:

Tail. The 737's sweeps back at two discrete angles -- shallower at the root, then steeper to the tip -- while the Airbus' tail rises directly from the fuselage without incident. You've got it dead to rights on that detail alone.

Wingtips. The A319/320/321 all sport Airbus' signature winglets, whereas 737-400 and earlier series have unadorned wingtips. However, the upturned winglet found on the 737-800, flown by ATA, is new twist that makes it a dead giveaway.

Cockpit. There are two "porthole" windows above the pilot and copilot viewports on 737s -- a classic Boeing design. Airbuses have somewhat wider-looking main panels, but no telltale portholes.

Engine nacelles. On today's most common 737s, the engine body is not perfectly round but takes a more elliptical, almost flat-bottomed profile, especially when seen from the front. The fitting on the Airbus is much more cynlindrical.

Extra Credit: Telling Airbuses apart. If you really want to knock your Premier Exec 1K VP's socks off, keep this handy Airbus field identification tip handy: A320 has two overwing exits; A319 has just one. Unless you see two of them parked side-by-side, you won't likely be able to tell a 319 is a 319 any other way, even though it is a stubby little mutha. A 321 is a stretched 320 with no overwing exits, but 4 full-size exit doors. Huzzah!

3. A320, meet 737. 737, meet A320
Note the acute angle that steepens about 1/3rd of the way up the leading edge of this B737's tail. Note: no winglets.	These frontier A319s showcase the simple pleasures of nature. Meanwhile, their tails meet the fuselage almost straight-on.
It's hard to do this point justice, but this photo sort of reveals the flatter bottom and elliptical shaping of the B737 nacelle. Sort of.	On this A321, the nacelle is pretty much flower pot-round. Also note that snarky winglet.
Airbus A319. No "portholes" above the main cockpit glass.	Boeing 737-300. Telltale "portholes" above attentive-looking German in orange jumpsuit.

The 757 is an airplane I fly all the time on United, but damned if I can tell one from the ground. What's the deal there? What am I missing? The 757 is an ungainly stork of an aircraft on the ground. It seems to prop itself up on hopelessly frail undercarriage when trundling about the terminal, but once those are folded up I think the 757 cuts a nice profile in the air (and offers a smoother ride, even in the rear, than smaller fowl like the 737 and A320). Here's how to spot one from the ground without a second's worth of doubt.

4. The thin-is-in 757, and the baby-got-back 767

The 767 has, I dunno, kind of a bullnosed-quality. Plus, if seen on the tarmac, the wings appear to span about a quarter mile. They're huge.

5. Old Fogey Tri-jets

In this profile view of a Canadian L1011, the intake's smooth incorporation into the body and tail of the aircraft is apparent. An S-shaped duct directs that airflow into the body frame-mounted tail engine. Beauty, eh?

Note how the tail-mounted engine mounts independently of the body of the aircraft. And they said it couldn't be done.

757 vs. 767 The 757 has, pound-for-pound, the slimmest shape of any other plane of its scale. If you look up and glimpse a very long and narrow shape with two engines on the wing, smart money says it's a '57. 757s never look stubby or compact and just a few glances at a 767 next to a 757 plainly reveal the 767 as a much larger, fatter (and depending on class configuration, phatter) aircraft. Look at the cockpit; there's a lot more cheek fat below the 767's viewports. Meanwhile, the 757's underbody sweeps back almost horizontally below the nose.

One other point on the 767 vs. the 777: They would be quite hard to tell apart except for one indisguiseable fact: the 777 has six wheels on each main landing gear truck; the 767 only has four. I think the 777's gear assembly is about the size of a Hummer H2, but it gets about 6mpg better on the highway.

I'm old school. But I'm also gettin' old. I can't tell an L-1011 from a DC-10. Planespot them for me, dawg. Back to the topic of tail-mounted engines raised in our 727 coverage: The Lockheed L-1011 (probably one of the best numeric names in commercial aviation) and the DC-10 appear quite similar on the ramp of your favorite airport. I say this because neither plane is still in service with a major carrier, but I do see a fair number of DC-10s in FedEx livery. Anyway, telling L-1011s from DC-10s is easy: you won't see an L-1011 unless you're flying a notable carrier like "Hewa Bora Airways" or "Kampuchea Airlines." Yikes. That and the telltale configuration of the aft engine. As you can see in Figure 5, the DC-10 configuration puts this engine body in the tail proper, while the L-1011 incorporates an "S duct" intake to a powerplant that's actually part of the airframe proper.

I hope this information at least makes you take a second look outside the next time you're killing time at the gate. Let me know if you have questions not covered here! I do requests. In a future posting I'll tackle the Airbus A340 and several flavors of 747.