Supersonic DC-8 (intentional)

Makes me wonder. The gauges are pretty unreliable from about mach .92- mach 1.05, jumping all over the place as the pitot system deals with transonic flow. Have been supersonic dozens of times from 100' AGL to FL 495 and the pitot static system was so unreliable in that transonic region that we had a system for dropping bombs in that region that threw out air data and just used INS data. Normally an INS-baro mixer was used for best accuracy.

Also, I assume they were talking about slats and not slots being damaged. A damaged slat locks out the flaps on some aircraft. Not sure how it works on a DC-8.

 

I remember hearing the 727 could do it too. Nothing to back it up though...

 

Hard to say. Some aircraft have no reaction to going supersonic except for the instruments fluctuating and the mach meter going over 1.0. If I were them, I would have gone a bit quicker to ensure they were not seeing any instrument error. Mach 1.01 is pretty close to mach 0.99. Mach meter uses both airspeed and density/temperature to calculate its readings. All were changing rapidly in the dive and fluctuating in that transonic region.

They had problems recovering, though, with controls in transonic flow (some surfaces supersonic, some subsonic) not reacting normally. Going faster would have increased those recovery problems. WW-II fighters hit what they called compressibility during extended dives. What they were actually seeing was transonic flow over some control surfaces that made the aircraft difficult to recover until lower altitudes were reached and the denser air caused the flow to go subsonic. Early jet fighters and the Me-163 would see mach tuck from the same effects.

 

Here ya go!

"TWA Flight 841 - April 4, 1979

"During a flight from JFK International Airport to Minneapolis-Saint Paul International Airport, TWA 841, a Boeing 727, deployed one of its slats while cruising at 39,000 feet. This, as some of you will know, is very bad. Not many of you are likely to know why, however, so permit me to explain.

"A slat is not a million miles away from a flap, literally and figuratively. A flap is a device on the trailing edge of a wing, which when deployed extends backwards from the wing and increases the lift that it generates at low speed. It is thus categorised as a high-lift device. The trade-off is that high-lift devices increase drag. A slat is another HLD, but is on the leading edge of the wing. It extends forwards when it is deployed, and allows the wing to continue generating lift at higher angles of attack, when it would normally stall. Again this is useful in low-speed situations like takeoff and landing.

"However, if slats are deployed at cruising speed they can cause violent oscillations, and in this case only a single slat deployed on a single wing, which resulted the aircraft making a sharp roll to the right. The air loading the slat was subjected to made it impossible to retract and despite attempts at correction by the autopilot and the captain, the aircraft went into a spiral dive and lost over 30,000 feet during the next minute. As a comparison, I would estimate a typical high controlled descent rate for a passenger airliner to be 5,000 feet per minute or less. Analysis of the flight data recorder by the NTSB indicated that the 727 exceeded mach 1 during the dive, and it was only after aerodynamic forces had torn off the errant slat, and the captain deployed the landing gear for some extra drag, that control was finally re-established with less than 10,000 feet to go.

"The aircraft was substantially damaged by the dive: the landing gear was damaged, parts of the flaps and the wing spoilers were missing and a raft of parts were missing or damaged, including a cracked cabin window. Despite that there were no further problems and once control was re-established, the crew made an emergency landing without incident. The investigation later concluded that there was no evidence of mechanical failure, and that the most likely cause of the incident was incorrect operation of the slats by the crew, which they strongly denied.

"The 727 was repaired and returned to service later the same year."

airliners that have gone supersonic and survived (idea) by archiewood - Everything2.com

Jacob

 

Blaming the crew for that one seems kind of stretched. You could accidentally lower the slats, with great effort for it to be accidental, but you cannot extend one wing's slat or slats accidentally.

I have seen adverse roll from the slats, but that was caused by bird strike damage (Plovers) that slowed the extension on one side. Normally, if they extend at different rates, they lock out, but not this time.

 

I believe that this has been discussed before but maybe it would be interesting again. The incident occurred (as told to me) when the pilot exercised a known 727 technique to increase lift at high altitude cruise by deploying the trailing edge flaps approximately 2 deg. This simulated the super critical airfoils that are in use today. To do this , however, the leading edge slats had to be separated from the TE flap operation because they were locked together so that when the TE flaps were deployed, the slats were activated. The LE slats were inactivated by pulling the circuit breaker to separate them from the TE flaps.The flight engineer had been in the lav and when he returned , he saw that the breaker had "popped" so he pushed it back in, thus energizing the slats at high speed. I was involved in the 727 from the beginning and trained over 2000 people inside and outside of Boeing in what the airplane was and what it could do and I was present when it was designed. It not only is a slick airplane but it is tough.
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^i remember this coming up once before already and that explanation of the events.

 

Bob- Makes perfect sense and the high speed could have damaged one slat and not the other until it blew off. That would account for the roll. Poor crew coordination for not letting everyone know what configuration was being used and why the circuit breaker was pulled. Must have been a pretty hairy ride. Diving barrel rolls does not sound like much fun in an airliner.

 

Thanks. When I was conducting tours for visiting flight crews and customers , I will always remember the AAL pilot looking at the 727 mock up with all the flaps, slats, and slots displayed in the dirty. He said, " You guys will never get all this s**t to work at the same time !" Others said that it was going to be a maintenance nightmare and highly unreliable but it proved to be one of the durable systems on the airplane. Looking back at that airplane always makes me realize what a great piece of engineering and aerodynamics it was. I can't think of anything that was a problem except the number 2 intake duct that was quickly and permanently fixed with some VG's. Removing all the struts and stuff off of a beautifully designed wing made it work to perfection. The hydraulic guys wanted a 4 system arrangement until the reliability guys stepped in and told them that they just increased the probability rate of failure so they stopped it at three. It was an exciting program in which to work.

 

DC8s did indeed have Slots and not the movable LE Slats we are more used to. One of my favorite airplanes to work on, especially after the CFM56 mod.

 

Bob- The 727s had a tough time taking off here in Albuquerque in the summer, when density altitude was usually around 8000' or above. I remember them shutting down part of whatever powered the air conditioning because they could not afford to lose the bleed air. Take-offs to the east from here were followed by a turn to the south to gain altitude to get over the mountains. The new 737s just climb straight over them. Flight attendants really preferred the 727 galley to the 737 galley, back when they were still serving meals, at least in first class. Back then my best/worst year flying with Delta was 177 segments, so got to see a lot of 727 take-offs.

 

I imagine that the JT-8D's were a bit challenged at times and I think that air conditioning bleed is shut down on take off on most commercial flights that I have been on lately.