Those who have been following the blog will recall the chilling video that formed the nucleus of the post, 'The Fatal Stall'. In this instance, the aircraft involved was a light aircraft, but in recent times the tragic demise of Air France 447 has reinforced that size is not a barrier to the lethal nature of the stall. And yet, despite the publicity, Air France 447 was not the only airliner to fall victim to the edge of the aerodynamic envelope. Below is the story of another.
"Even the Mighty Can Fall"
The Boeing 727 is one of the all-time classic airliners. Built for speed, it is a pilot’s aeroplane that offers a hushed ride for its passengers by virtue of its three aft-mounted Pratt and Whitney engines which left most of its noise in its wake. Yet even such an illustrious machine can fall victim to the simplest oversight as the crew of Northwest Orient Flight 6231 learnt on a cold winter’s night in 1974.
Prelude to disaster.
The sector should have been little more than a milk run. The flight was a short positioning sector from New York’s JFK Airport to the upstate town of Buffalo where an American football team and its staff were awaiting a ride back home. The crew of three were the only occupants for the ferry flight on the evening of December 1st and consisted of the Captain, First Officer and Flight Engineer. The Captain had held a command for five years, while the F/E had around 2,000 hours in the back seat. The First Officer was to fly the sector and had previously been a Flight Engineer before changing to a ‘window seat’. Of his 1,500 hours as a pilot, only 50 of them were on the 727.
As the crew readied the Boeing, the forecast for the night ahead was typical of a cold winter’s eve on the east coast. The cloud base sitting at around 5,000 feet with occasional thunderstorms extending towards 30,000 feet and icing virtually assured for all levels in between. The crew had planned to cruise above the weather at 31,000 feet and had very little reason to believe that the flight would be anything other than routine. And yet in less than 15 minutes after departing JFK they would crash to earth in a forest a mere twenty miles to the north. But how?
Air Traffic Control’s first indication that a problem existed for Flight NW6231 came in the form of a Mayday call stating that they were, “...out of control and descending through 20,000 feet”. In response to ATC’s transmission, the crew’s final message was that they were, “...descending through 12....we’re in a stall.” In less than a minute they were dead.
The Boeing had hit the earth at high speed and its wreckage was confined to an area less than 50 metres square. There were some tailplane components a short distance away, but it was evident that these had separated in flight due to high aerodynamic loads. The undercarriage was retracted, but the leading edge devices were extended. While a number of the aircraft’s pitot heads had been damaged, two of the airspeed sensing systems were found to contain water and began to point the finger at possible icing issues. This was further reinforced when closer examination of the flight deck found the overhead ‘pitot heat’ switches were in the ‘OFF’ position.
Much more would be revealed when the aircraft’s Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) were analysed. As the flight was so short, both recorders contained voice and data recordings from before take-off to right up until impact and both portrayed a relatively normal flight prior to 16,000 feet. At this point the aircraft was established in the climb at 300 knots and climbing at 2,500 fpm enroute to cruise at FL310. Incredibly, the airspeed began to climb through 340 knots while the climb rate exceeded 5,000 fpm. The First Officer was taken aback by the increase; however the crew put it down to the aircraft being light and possibly an updraught associated with the forecast thunderstorms.
Incredibly, as they climbed the indicated performance was astronomical with a speed of more than 400 knots and 6,000 fpm rate of climb as the aircraft punched through 20,000 feet. Still hand flying the aeroplane the First Officer was continuing to pull back on the control column in a vain attempt to arrest the growing airspeed as the overspeed warning sounded. The Captain encouraged the new co-pilot to keep pulling back but it was all to no avail as the overspeed warning sounded again. And then, only seconds later, the stick-shaker activated to warn of an impending stall.
On the verge of 25,000 feet, the crew were still convinced that they were flying at excessive speed and rationalised that the stick-shaker was in fact Mach buffet at the other end of the performance envelope. The Captain again told his co-pilot to “Pull it up”. The gear warning horn chimed in, indicating that the gear was retracted while thrust levers were closed and idling at the stops. It was now all too much for the 727 and the airliner lurched from its nose high attitude to more than twenty degrees down as it simultaneously turned to the right through 180 degrees to point back in the direction of JFK. In an instant its rate of descent increased to 15,000 feet per minute. Yes, 15,000!
The aircraft continued its downward plunge and at around 12,000 feet the Captain recognised that the aircraft was stalled and called for “Flaps Two” as the stall warning continued intermittently. Ultimately the aircraft was descending with a 50 degree pitch down, 80 degree roll to starboard and a rate of descent of up to 18,000 feet per minute in association with G-Forces reaching 5G. Ironically. By this stage the airspeed indicator was reading zero as the tailplane partially failed under the load. The crew never stood a chance at this point and their fall from over 24,000 feet had taken a minute and twenty seconds.
The attitude indicators were frozen at 20 degrees nose down at the point of impact, but it was what they were indicating a few minutes earlier that was crucially overlooked. Climbing through 16,000 feet when the incredible climb performance first began to accelerate, the pilot’s action was to increase the back pressure in an attempt to arrest the blistering speed. If due attention had been paid to the aircraft’s attitude, the nose was actually more than 30 degrees up, when a more likely attitude was in the realm of five degrees. Continuing to pull back only exacerbated the issue until the critical angle was exceeded and the aircraft stalled. So began the aircraft’s rapid fall to earth, but even so, there was no attempt to roll off the high bank angle as the aircraft descended so that any back pressure on the control column merely served to increase the G-loading. Everything was working against the crew possibly recovering the aeroplane.
But why would the crew receive such phenomenally high airspeed indications when in fact the aircraft was stalling with a high nose attitude and the thrust levers were closed? The simple answer is that the pitot heat had not been selected prior to take-off and the multiple probes had iced up until they were blocked passing 16,000 feet. The CVR revealed that there had been some hesitation, confusion and oversight when the pre-take-off checklist had been read, with the ultimate result that the pitot heat was not selected ‘on’. Such a simple error, but such an extreme result. The indicated airspeed was far from accurate once the system was blocked to the extent that when the aircraft stalled at 24,000 feet it was indicating over 400 knots but flying at less than 170 knots. Conversely, in the final stages of descent when there was zero indicated airspeed, the aircraft was probably flying in excess of 350 knots. Without due attention to attitude, confusion undoubtedly reigned supreme.
The Lessons Learnt.
For such a tragic outcome, the findings revealed that it was the failure to successfully complete a checklist that created the problem and an undue focus on airspeed rather than aircraft attitude that led to disaster.
The need for checklist discipline, whether for the lone pilot or the airline crew, is absolutely vital. Cockpit interruptions are frequent and distracting, but shouldn’t circumvent crucial checklists. If the flow of a checklist breaks down for any reason, there is a strong case to go back to the beginning and start it again. And then, don’t stow the checklist, move the marker or flip the page until the checklist is absolutely completed. Even if flying solo, recognise this point by stating out aloud, “Pre-take-off checklist complete.” It provides a further filter and is another marker in the brain that the job is done properly.
As for flying an aircraft with suspect instrument indications, the first thing is to consider whether the attitude and thrust setting is appropriate for the indicated performance. Confirm that the pitot or probe heat is selected to ‘ON’ and cross-check the Mach/airspeed indicators against each other as the fault may not lie across the entire system. First and foremost, consider the attitude and thrust in light of the performance.
The investigators did not find any great mystery in the loss of Northwest 6231. A simple oversight here and a misinterpretation there led to the catastrophic loss of an essentially serviceable Boeing 727 and its precious crew. Regardless of the size of the aircraft or the experience of the crew, the operation of any aeroplane is hinged upon discipline and the observation of some fundamental principles. This is sometimes easier said than done, so it is our responsibility as aviators to give due attention to every aspect of our operation and guard against the curve-balls that fate throws at us.
That cold night over New York the unfortunate crew saw events unfold at a rate that exceeded the ability to recognise what was befalling them and it all stemmed from failing to flick a switch. We are all only human and there but for the grace of God go I.
Title Image: r2suberti.blogspot.com
B727 Image. The extensive 'Ed Coates Collection'