Advanced Aircraft Performance: Understanding Coffin Corner

Discuss about the Advanced Aircraft Performance.

The concept and phenomenon of the coffin corner has been a concept that has been unaddressed completely with proper and potential solutions. The continued accidents, for larger and commercial flights have been the causes for loss of many lives and through the challenges to the design of the aircraft and the operation of the aircrafts, if they accidentally enter the stall regions.

Air France Flight 447 was a passenger flight that travels from Rio de Janeiro to Paris and France via Brazil. This flight was unfortunately crashed on June 1^st, 2009. The flight was run and operated by Air France (Rapoport, 2011).

The flight was crashed after it entered an aerodynamic stall and later crashed and fell into Atlantic Ocean and killed the entire people, who aboard the flight, cabin crew, aircrew and total 228 passengers. So, the flight has entered high altitude stall and later impacted ocean.

According to the final report of BEA (Bureau d’Enquetes et d’Analyses pour la Securite de Aviation Civile), the crashing of the aircraft was done, after temporary inconsistencies that happened in between the measurements of airspeed. The inconsistence might have happened likely because of the pilot tubes of the aircraft, which were obstructed because of the ice crystals. This has caused disconnection to the autopilot. Eventually, the crew has responded and reacted incorrectly. It allowed the flight to enter the aerodynamic stall and it was made impossible to recover from it.

Flight 447 flew at the altitude of 35,000 feet, where the relationship between the stall speed of the aircraft and the sound’s speed has got the names called ‘the coffin corner’. The link here is about the shape of the plot of velocity of stall versus. At this point velocity is considered in terms of Mach number, which is the speed relative, the sound’s speed. The link here is only about the shape of plot, but not the meaning of ‘deadly to fly’.

Though there have been many incidents, where the coffin corner incident was occurred by many aircrafts, things were in control and the accidents were resulted because of the other failures and loss control. However, the threat of coffin corner cannot be violated or neglected, which can be serious prone to the aircraft crashes.

The concept of coffin corner has other names called Q corner or aerodynamic ceiling.

Figure: Coffin Corner

Coffin Corner

The concept of coffin corner can be understood as the altitude, at which stall speed of the fast fixed wing aircraft would be equal to the critical match number, at specific G-force loading and gross weight. The flight would be very difficult to stand in stable state at this altitude. Here, the flight has to maintain its minimum and maximum speeds based on two constraints. The minimum speed is the stall speed, so that the flight can be maintained without falling down by losing the altitude. The maximum speed is the critical mach number, which is the maximum number at which the air does not lose lift and travel over the wings, because of the separation of flow and shock waves. If the flight increases more than this speed, the flight starts losing lift and lose altitude, by pitching heavily nose down. Here, the word corner, from the coffin corner refers to the shape of triangle at flight envelope’s top chart, where critical mach number ad stall speed are joined at this point (Jonathan, 2010).

Since, the minimum and maximum speeds of the flight are very well associated with the coffin corner, the concept of coffin corner has become vital and essential to follow for the flights for stable movement, without falling or losing its altitude.

High Altitude Upset is an important implication for regulated performance requirement for safer operation of the large crafts. Upset is interpreted as a loss of control, caused from stalling. The flight envelope, at higher altitude, the scope to increase the altitude or change the velocity is restricted greatly. It is caused from the thin air at altitude, which in turn gives two effects (Jonathan, 2010).

The first effect is that the sound becomes will be at higher altitude

The second effect is the stalling speed of the aircraft would be more in such thin air

So, if the flight continues to fly straight and gets levelled at higher subsonic speed, in such increased altitudes and the pilot tries to accelerate in such conditions, the flight moves close enough to sound’s speed and buffeting or sound barrier could be resulted. If the flight is tried to slow down, it would be easier to slow down to reach the stall speed of it. Then also the pilot would start feeling the buffeting, because of the stall effects. Buffet is the huge and dangerous feeling of vibration that could even be reached to 0.2g.

Implications Of Operation Of A Large Craft

On the other hand, if the pilot attempts to move higher and climb upwards, at higher altitude and subsonic speed, buffeting can be induced, because of the increased attack angle in the thin air. This concept is coffin corner and it is not an exception for the modern flight.

The performance data of the coffin corner experiment is confined largely confined to the aircrafts that are experimental and under test conditions. However, the data shows that the coffin corner has been affected to the commercial flights, such as Aircraft France Flight 447, Aircraft 330 and Pan American Boeing 707 (Jonathan, 2010).

Figure: Diagram to show the difference found between Pitch angel and Angle of attack

Angle of Attack is considered as an angle found between te chord plane of the wing and the direction of travel of the plane. AoA is an important consideration to determine the stall speed. Pitch angle is considered as the angle in between horizontal and fuselarge centre line. The major difference found between the Pitch angle and AOA is that AOA, which could prevent the stall, cannot be considered as a feel that can be felt by the pilot, as the pilot is dependent on the instruments (Thompson, 2013). But the pilot can have at least some awareness of it, as it could affect the feeling of the pilot. However, Pitch angle cannot be considered as an important parameter to avoid stall.

When Air France 447 is considered, the accident was subjected to extremely detailed and reported by the authorities of France. It was a bizarre accident, in which one of its pilots behaved strangely, so was unaware of what the pilot was doing, because of freezing in panic completely. Unfortunately, the other pilots war unaware of the condition of this pilot and cannot interpret properly for the instrumentation. The result is the vanish of the aircraft and it recorded no Mayday calls, as the flight was landing in mid-Atlantic and no radar records were made available, as it was in the center of the ocean (Jonathan, 2010).

Almost after two years, from the accident, in 2011, recovery of the cockpit voice recorders and light data recorders were found. The accident was caused by the pilot probes, which was caused from the ice crystals. Then the automatic system was disconnected and there were incorrect speed indications shown. Though the captain and co-pilots were re-joined, it was after 1 minute 30 seconds, however, the flight went into stall situation. It fell from 35,000 ft. and within four minutes of time. The flight was perfectly alright with no mechanical or electrical malfunctions.

Flight Performance Data

The Air France 447 accident is not influenced by the fuel consumption. Ideally, the amount of fuel can be as much as possible, however, it would depend on the weight and balance of the flight. Calculation of the fuel requirement for the aircraft depends on various variables and it legally depends on the fuel reserves needed for the regular trip and additional reserves that include diverted travel.

Airbus gave certain recommendations for changing the Pilot tubes model that are installed in A320, A340 and A330, in September, 2007, because of water ingress problems. However, Air France attempted to decide for replacing the A330’s pilot tubes, only in cases of failure and so it was not airworthiness directive. However, there have been the situations, in 2008, where airspeed data are lost during the flights, because of icing of pilot tubes, though it was temporary. Then Air France started accelerating the replacement programme for the Pilot tube. This program was implemented from 17^th June 2009 (Rapoport, 2011). Later, the recovered cockpit voice recordings and recovered flight data recorders were enabled to record the details of what was happened to work out (Thompson, 2013).

Loading solutions for aircrafts can now be done with the use of the technology, using software. The loading solution of the fuel is not an issue for the aircrafts addressing now.

According to the training of the pilot, the reaction after approaching the stall, the controls are to be moved or pushed forward. There are two sticks, both sides of the pilot and they act as game controllers. Both the pilots have these two sets of sticks and they move independent to each other. So, non-flying pilot does not the actions performed by the flying pilot (Rapoport, 2011).

The coffin corner and stalls situation have to be well handled to ensure that the aircraft does not get affected by the altitude and control.

Pilots have to be trained sufficiently to control the flight, in high-altitude stall recovery.

The angle of attack has to be inferred indirectly with reference to the speed, towards recognizing the stall and recovery.

The human machine interface has to provide the information that is unambiguous and clear, especially in the fault conditions.

Too many alarms should not bombard the pilots.

Sufficient training has to be provided to the pilots to ensure that they consistently maintain situation awareness. So, they should consistently retain a better mental model of the machine-system’s state.

Operations of a Large Aircraft

Since the pilots are not engineers, they by default have to believe the data displayed and presented to them.

All the above solutions have not been addressed, especially, there is disconnect between the anticipation of the design engineer, about the rational and irrational behavior of the pilot and the design of the aircrafts. Eventually, the design aspects cannot be in such conditions that the aircrafts would continue to function and in control, irrespective of the irrational behavior of the pilot operator (Jonathan, 2010). Though many of the situations, handling stalls in coffin corner conditions has been addressed to some extent, there is no complete solution developed and implemented in the overall design and structure of the aircraft even in the modern aircraft design.

So, it cannot be said that the modern aircraft is not susceptible to the coffin corner phenomenon. There is no assurance from the designer till now that there cannot be danger of stalls and coffin corner for the modern aircrafts (Thompson, 2013). Though it is high performance aircraft, it can be concluded that it is not exception for the occurrence of the coffin corner.

Since the larger aircrafts, which have the probability to get into the stall area, through coffin corner, have to be built with the solutions. They are recommended to build the flights to travel within the specified limits. In addition to that, the pilot operators are to be well trained against operating the airplane safely in the coffin corner.

Conclusion

Air France 447 has been suffered from the coffin corner phenomenon. The same phenomenon has been occurred for another aircraft, A330 and Boeing, before its occurrence. Eventually, the phenomenon of coffin corner has come into a wider concept for discussion. The concept of the coffin corner has been experienced in the way that the aircraft enters into the stall and loses its control. When the aircraft enters into the stall, control of the aircraft becomes difficult, because of the challenges to maintain both the minimum and maximum speeds. Eventually, there are many implications resulted in the requirements of the regulated performance implications for the operation of the aircrafts. The performance of the flight becomes uncontrollable, no matter it is a small flight or larger flight. Performance of the flight should be in control, before entering the stall and it should be very well controlled, during the operation of the flight by the operators. The solutions are proposed, based on the experiences gained from the failure of the Air France 447. It is concluded that, since all the implications and challenges of the stalls and coffin cabin phenomenon are unaddressed completely to resolve, the modern larger flights are still susceptible to the phenomenon of coffin corner.

References

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