Passed through Salt Lake City a few days ago and noticed the largets gathering of MD90's ever. I noticed thats almost all of them share a similar birthmark if you will. On the top back half of the engines, I noticed that on almost all of the md90s there is a brown/black trail of dirt and debris on the back half of the engines. I know over time aircraft get dirty, but even the aircraft painted in the 2007 Delta scheme have this same dirt on the engine. What causes this?
Pictures to better describe what I am talking about
Photos: McDonnell Douglas MD-90-30 Aircraft Pictures | Airliners.net
Photos: McDonnell Douglas MD-90-30 Aircraft Pictures | Airliners.net
Photos: McDonnell Douglas MD-90-30 Aircraft Pictures | Airliners.net

 

On the righthand side of the fuselage just below the tail is the APU exhaust. On top of the engine is probably caused by the bucket type thrust reverser.

Denny

 

It is a cascade reverser system. What you are seeing is junk gathering in the the hyd residue on the aft part of the cascade on the reverser...

 

Why don't we see more 'clamshell' type reverse thrust systems on large aircraft? They're more effective, and, to my basic reading, much less mechanically complex (cascades even take a cruise thrust penalty, from what I've read). The main downside that I see is, of course, if a clamshell opens in flight - that's a bad day, though the DC-8 used them in flight, so . . .

Is it not feasible to have a large, high-bypass ratio engine with clamshell reversers? Is the installation of them on underwing engine mount designs more difficult, as I seem to see them on modern T-tail business aircraft (and, again the DC-8 had them on underwing mounted engines). I can see why it wouldn't work on the GE or Pratt configuration, but RR seems to prefer to shroud their entire engine (rather than having the core stick out the back of the bypass area), which would seem to allow for an easy installation of clamshells. I know the ERJ's have RR's and clamshells, and I assume, given the age of the ERJ, the engines are of a rather high bypass ratio.

Anyone know the deal? Is it is fuselage size issue? A size issue with huge engines and high bypass design? An aero issue with underwing mounting?

 

Ground clearance could be an issue. On the MD-88 and DC-9 line of aircraft, there have been many clamshells that have made contact with the ground while opening them with the nose off the ground.

 

Ground clearance as posted is the issue. With the nose gear still off the ground or with a wing down landing clearance on a big high bypass engine can be tight.

 

My guess would be that it's easier, cheaper and more reliable to deflect cold fan air than the hot exhaust gases. Since hi bypass engines develop more thrust from the fan air than the core, you're not losing much by blocking the fan.

 

Are the V2500 engines the same ones United and USAirways uses on their Airbus aircraft

 

You have come correct... just a different thrust rating and mounting.

 

Yet another design factor discovered by the operator...

But at least when they are open they destroy the airflow over the rudder, reducing directional control.

Douglas Jets, built in Long Beach, engineering done in the field.