MD 80

[hanspetter]

There's been a lot of focus on the MD 80 series after the Madrid accident. Yesterday the CEO of "Norwegian" stated that they have avoided MD 80s because they require too much service. Norwegian got some MD 80s when they bought "Fly Nordic". I actually flew one of those from Umeå to Stockholm a few weeks ago. Anyway, Norwegian wants to avoid anything but B737s in their hangar it seems.

Questions, which are the design features of the MD 80s that may require more maintenance that comparable medium-range airliners? Looking at what has gone wrong in the past I see no obvious common denominator except one: Engine fires may involve the cabin a lot faster when the engines are mounted on the fuselage. Actually, there's one more, laterally fuselage mounted engines are more likely to incapacitate rudder / elevator if they blow up as the left one did in Madrid.

On the other hand, an engine out situation creates less yaw with fuselage mounted engines. The engines ride higher and are less prone to picking up debris on the runway.

However, we should always keep in mind that what may seem like an accident-prone aircraft may simply be a very common aircraft. If all airliners were B737s the B737 would be the most accident-prone design.

[Tom Goodrick]

I think a big factor is ice shed from the wing root area going into the engines. A little of that every once in a while can require a lot of inspections. For an airliner like the MD-80, ice is a year-around problem.

The engines on the 737 would be imune to injestion of shed ice.

[flaminghotsauce]

Wasn't the Alaska Airlines crash an MD80? That had the elevator control rod threads strip out.

I don't know, just my instinct is that it's a lesser aircraft than the Boeings. 737's 757's and 767's seem to be everywhere. The aircraft that made an emergency landing at KIRK a couple years ago was an MD90 or whatever the nomenclature is. It flew into a storm and caught hail for it. ;D

Sorry, BAD pun.

[hanspetter]

They told on the news that the pilots had forgotten to deploy "the so-called flaps". "These are supposed to give the aircraft more lift during a take-off". Will a flapless take-off explain this calamity? They may have been closer to a stall due to a premature rotation but my FS experience would indicate that it would pick up speed faster and become manageable. What do you think? Adding to my doubts, there were passengers reporting that one engine went out with a bang and that's got nothing to do with flaps settings.

[flaminghotsauce]

Yes it could be a factor. If flaps, er, "so-called flaps" are called for in the take-off checklist, then they were not following the checklist. Flaps will give more lift, so they are pretty common take-off item. If they didn't deploy, and the pilots saw the usual airspeeds and rotated, they would indeed be too close to stall. The idea is to have as much lift as practical to get the aircraft climbing. Lack of flaps would lead to a longer take-off run. Short field take-off procedures in the little Cessna 172 involve one notch of flaps. Jets doing a balanced-field approach to take-off length gain advantage with flaps.

This ignores the engine blow-up. It could have been a better outcome with both engines producing take-off thrust, but still dangerous if they ignore the flaps.

Now the engine popped after they were off the ground if I recall correctly. It makes me wonder if they were panicked at the weird take-off numbers, or how it felt, or something, and shoved the throttles to get more thrust? Just speculation, of course.

[Tom Goodrick]

While it is conceivable that lack of flaps could cause a crash, it is unlikely if proper procedures were followed. The flaps on a large jet airliner are very significant during takeoff and landing. I dug out the numbers on my version of the FD for the MD-83. They may not be highly accurate but they are reasonable.

Takeoff and stall performance both depend on the lift coefficent. With all the flaps retracted, and if loaded to max gross weight, the stall speed is 153 KIAS. With full flaps as used on landing, the stall speed is 110 KIAS. With half flaps as might be used on takeoff, the stall speed is 127 KIAS. Let's say they were expecting half flaps so 127 KIAS would be the stall speed. If they tried to lift off with a fast rotation at 140-150 KIAS, yes indeed, they would remain airborn only for a short time and then would crash. But if they followed proper procedure and lifted off at V2 - takeoff safety speed which is 1.3 x stall, they would lift off at 165 KIAS and be safe even if the flaps had not deployed.

One would expect that 140-150 kIAS would not even get them off the ground. But there is a dynamic effect on the wing lift of a rapid rotation that can keep the air attached longer than in steady flight so their may be enough lift to get airborn but not enough to sustain flight.

Of course one can see all kinds of scenarios where these numbers could get closer and the lack of flaps could cause a crash. I heard there was speculation the flap position indicator was malfunctioning.

In the US it takes at least a year for the NTSB to compile a report on a crash like this. There are always many factors to consider. The cause is seldom obvious. But if all the flaps are found retracted in the crashed wings, that is a clear indication flaps were a factor. But it may still have required the pilots to follow an improper procedure.

This FD can be downloaded from my web site and used on the AI MD-83 that comes with FS9.

[hanspetter]

Good explanation and a belated reply on my part
My muddled thinking caused me to assume that the aircraft would simply keep rolling on the tarmac if you tried a premature take-off. Hence, it would gain the required flapless speed before taking off and the risk would be limited to running out of runway. This is clearly not the case since we can indeed yank most aircraft off the ground and witness a few seconds of unstable flight until it veers and drops.

I noticed another report stating that the slats were extended but not the flaps. Further, the error might have been technical in that the cockpit indicator may have shown flaps extended when they were in fact stowed. The extended slats may be explained by an MD-80 feature that auto-deploys slats when a stall is imminent.

However, from an aerodynamic perspective, what's the situation if you run out the slats only? I envision a pitch-up but that may not be what takes place.

[Tom Goodrick]

I don't know what the effect is but it's too late after the slats deploy with a stall close to the ground. The only solution is to lower the nose considerably and trade altitude for speed to recover.

I made a takeoff on the numbers for flaps but with flaps up in the MD-83 and it worked fine. But I had a nice 10,000 ft runway. If the runway was short, there would have been a problem.

I must also say that my calculations and the behavior of my MD-83 may be sufficiently inaccurate to invalidate the solution I postulated (ie- using the proper numbers for takeoff). The factor of 1.3 over stall speed expecting takeoff flaps may not get you to a safe airspeed with no flaps. Then there are such variables as wind, payload and CG.