Sudden Onsets (Tail Icing or.....?)

On Oct. 31st, 1994, an ATR-72-210, American Eagle flight 4184, crashed near Roselawn, Indiana, killing all 68 on board. The plane was in a holding pattern in icing conditions, when it suddenly rolled over and dove into the ground.

Ice, forming on the airframe and in particular aft of the deicing boots---which are located on the leading edge of the wings---was the cause of the sudden rollover. The existing regulations, in respect to design certification of an airliner, were not adequate when it came to the ability of the plane to operate in light to moderate icing conditions.

Extensive airborne testing, following that accident, revealed it is possible for airliners to encounter water droplets exceeding 200 microns in average diameter, which is probably what happened to AMR 4184. The existing FAA design certification rules for Transport Category aircraft (Part 25, Appendix C), at that time, only required the plane to handle droplets with an average diameter of 20 to 50 microns.

The ATR was designed and manufactured in France by Aerospatiale.  Mr. Fredrick's  book, Unheeded Warning, reveals how political considerations overruled the safety mandate that is supposed to govern the FAA's design certification decisions on aircraft of foreign manufacture.

Fredrick details strong evidence that some experts in the FAA, knew the plane would be dangerous in icing conditions, but they were overruled by higher officials because the French might have been offended if certification was denied by the FAA. He also demonstrates that numerous "close-call" incidents and one accident (10-15-87, near Lake Como, Italy with no survivors), which preceded the Roselawn crash, were known to be the result of in-flight icing, but Aerospatiale effectively covered-up and did not circulate that information, lest the reputation of its ATR aircraft be damaged. The FAA also failed to follow up on the findings in the accident report of the Italian crash. If it had, the 68 that died at Roselawn might well be alive today.

On December 26, 1989, a United Express, flight 2415, suddenly pitched nose down, while on final approach to the Pasco, Washington Airport. The pilots, of that British Aerospace BA-3101 turboprop aircraft, were not able to recover and all died in the ensuing crash.

As in most accidents, there were multiple factors that combined until they culminated in a crash. At the previous enroute stop, the captain did not have the plane deiced; instead crewmembers manually removed some of the accumulated ice from the wings and tail. There was also evidence that the deice distribution valve, which controlled the amount of air available to the deicing boots on the leading edges of the airfoils, was partially defective and may not have allowed the boots to inflate to their full capability.

The NTSB concluded, in its final accident report, that during the descent into Pasco, the plane accumulated from 1/2 to 1 inch of mixed rime and clear ice and that it most likely took the "mushroom" or "ram's horn" shape that is very detrimental to airflow over both wing and empennage (tail) airfoil surfaces. Since the plane did not carry any "black boxes," (cockpit voice or air data recorders), the NTSB could not be certain of what control actions the pilots took, while on that fatal approach. However, the Board was able to come to a conclusion, with aid of computer simulation of the sudden pitch down (50 to 60 degrees nose down at impact), combined with the ATC radar tape (which recorded ground speed, course and glide slope tracks), that the plane stalled because of ice accumulation.

They couldn't be certain if it was a wing stall or a tail stall, but the most likely scenario was a tail stall, since the computer simulation showed a wing stall as producing only a 35-degree nose down pitch, whereas the tail stall would match the actual 50 to 60 degree down pitch. It matches the classic tail-stall profile that has been seen in other accident investigations over many years. It usually happens suddenly and without time for recovery (because they are too close to the ground on that stage of the final approach), when the flaps are extended to their maximum range.

On March 4, 1993, a Continental Express ATR-42 experienced a sudden roll, but the pilot was able to recover and make a safe landing. There were other such incidents, prior to the Roselawn crash, but they were passed off as turbulence encounters, even though icing conditions existed at the time. It wasn't until after the Roselawn accident that it became clear that those incidents too, had been near-disasters precipitated by the inability of that airfoil design to handle more than light icing conditions.

On Jan. 9th, 1997, an EMB-120 Brasilia turboprop, (Comair 3272) crashed near Detroit, killing all 29 on board. 

The NTSB  found the probable cause of that accident to be: 

...the FAA's failure to establish adequate aircraft certification standards for flight in icing conditions, the FAA's failure to ensure that a Centro Tecnico Aeroespacial/FAA-approved procedure for the accident airplane's deice system operation was implemented by U.S.-based air carriers, and the FAA's failure to require the establishment of adequate minimum airspeeds for icing conditions, which led to the loss of control when the airplane accumulated a thin, rough accretion of ice on its lifting surfaces. 

Contributing to the accident were the flightcrew's decision to operate in icing conditions near the lower margin of the operating airspeed envelope (with flaps retracted), and Comair's failure to establish and adequately disseminate unambiguous minimum airspeed values for flap configurations and for flight in icing conditions.

In March, 1998, a WestAir EMB-120 Brasilia, departed Sacramento and was immediately placed into a holding pattern in icing conditions. It was only in its second turn, in that holding pattern, when it suddenly rolled and dove towards the ground. The pilots immediately extended the flaps and were able to recover before they hit the ground. The pilots, in the Roselawn crash, also tried to extend the flaps, but the French computer, on that plane, would not allow flap extension because the computer determined the speed exceeded the limit for that degree of flap extension.

On March 19, 2001, a Comair  EMB-120 Brasilia, experienced an upset event after encountering icing conditions. The aircraft, flying from Nassau, Bahamas, to Orlando, Florida, made an emergency diversion to West Palm Beach, Florida. A ground examination revealed substantial damage to the elevators and the horizontal stabilizer.  Fortunately, the captain was able to recover from the dive and there were no injuries to the 28 onboard.  Comments from the NTSB summary:

The crew reported that the airplane's systems, including its ice detection and anti ice systems, functioned normally before the upset.

They indicated that the airplane was being controlled by the autopilot at about 18,000 feet when they encountered instrument meteorological conditions that rapidly led to the windscreen being covered by a layer of ice.

The crew turned ice protection systems on and the ice on the windshield was cleared. The first officer observed ice on the right wing's boots and the right prop's spinner that extended farther back than he had previously experienced. The first officer switched the ice protection systems to their highest settings.

The first officer notified the captain of a decrease in airspeed from about 175 to 160 knots.

The captain disconnected the autopilot, applied power, and initiated nose down pitch inputs to arrest the airspeed loss. They indicated that these actions were unsuccessful and the speed further deteriorated to about 130 knots at which point the airplane experienced oscillations about its pitch, yaw, and roll axes and subsequently rolled sharply to the right and entered a steep descent.

During the descent, the electronic attitude display indicators in the airplane were observed to intermittently present no useful information. The captain stated that, "when we needed it [the electronic attitude display indicators] the most we didn't have it."

The airplane descended between cloud layers into visual conditions where recovery occurred about 10,000 feet.

After the recovery, no anomalies with the airplane, its handling characteristics, or its systems were noted.

The flight data recorder and the cockpit voice recorder have been examined at the Safety Board's laboratory. The cockpit voice recorder continued to run after the landing and did not provide any useful information regarding the upset. The accident airplane's solid state 25-hour FDR captured the event and functioned until power was removed on the ground. Preliminary review of the FDR data indicate the following sequence of events:

The airplane was at about 17,000 feet, with the airspeed stabilized around 200 knots indicated airspeed (kias). The autopilot was engaged.

The airspeed slowed from about 200 kias to 180, and the airplane began trimming nose-up. The airspeed continued to decrease to about 140 kias while trimming to a nearly full nose-up position.

The autopilot disconnected and the airplane rolled about 90 degrees to the left, and then back to near level. In the next 24 seconds, the airplane again rolled about 110 degrees to the left, back to level, then about 120 degrees to the right, back to level, and then rolled 360 degrees to the right, back to near wings level. Since the crew reported trouble with the flight attitude instruments, the roll angles recorded on the FDR are being further investigated.

The maximum nose down pitch attitude was 60 degrees, the maximum recorded airspeed was about 240 kias, and maximum vertical acceleration during recovery was about +3.6 g.

In April, 1996, following the Roselawn crash, the FAA issued 18 new airworthiness directives (ADs) affecting 29 models of turboprop aircraft. Those aircraft all have the same common features in their designs:

---- Unpowered flight controls.

---- Pneumatic deicing boots.

---- NACA five-digit sharp-stall airfoils (which were made obsolete by the more modern soft-stall designs).

The ADs require extensive instruction, to pilots flying the affected aircraft, on how to fly in freezing rain and drizzle (including the prohibition of the use of the autopilot in icing conditions), how to recognize indications of severe icing, and then require an immediate exit from icing areas. In addition, both ATR-42 and ATR-72 aircraft had their deicing boots modified to extend the boot area to reach back to 12.5 % of the chord. Previously, they had extended only to 5 % and 7 %, respectively. In theory, that should solve the problem of the tendency of ice ridge formation at the 9% chord position of those obsolete sharp-stall airfoils.

However, it still doesn't deal with the results of the Bascombe-Downs tests, conducted by the British, which demonstrated ice could form as far back on the wing as 23% of the chord, and on the tail at 30% of chord. Both percentages remain well beyond the limits of the deicing boots. Those tests limited the size of the droplets to 40 microns, near the maximum limit of the archaic FAA design certification rules for Transport Category aircraft (Part 25, Appendix C), still in effect at that time of the Roselawn crash.

That is why I still believe that some, if not all, turboprop airliners still have a serious problem in regards to tail icing. To the best of my knowledge, nothing has been done to make tail deicing more effective.

STEPHEN A. FREDRICK, the author of Unheeded Warning, came very close to "buying the farm," when he was flying the ATR in icing conditions. After the Roselawn crash, his conscience compelled him to work surreptitiously to expose the deficiencies of the ATR aircraft, in icing conditions, and the history of how those deficiencies were known and covered up by the French manufacturer, the FAA and airline officials. Fredrick is a rare person of genuine conscience. He was willing to give up his job as an airline pilot (actually -- his entire career, since he cannot hope to ever be hired again by any airline), to expose the facts about the ATR.

My recommendation to the flying public is the same as that of the courageous American Eagle pilots who wrote and distributed an anonymous pamphlet at Chicago's O'Hare airport, after the Roselawn crash (Fredrick was one of those pilots):

"...If the weather is clear this winter, sit back and relax because this is a good aircraft. If the weather is cloudy, snowy, or cool and rainy, think about alternate transportation methods..."