BUFFET BOUNDARIES- COFFIN'S CORNER

Stall speed is defined as the minimum airspeed required to maintain 1g level flight. Any further reduction in speed will result in the lift produced by the wings to be less than the weight of the aircraft and leads to a loss of altitude. The increase in angle of attack will in turn cause flow separation from the upper surface of the wing . In a swept back high speed aerofoil, this flow separation and associated pitch down will not be a marked phenomenon. Instead the aircraft enters into a descent. The descent rate further tilts the relative airflow downwards and leads to an increase in angle of attack further driving the aircraft into the stall regime. Any attempt by the pilot to raise the attitude by aft pressure on the elevator will cause a further increase in angle of attack and further loss of altitude.

The lift, however, depends on both air density (kg/m³) and on the plane’s velocity, and air density decreases with altitude. So, the higher you go, the faster you have to fly to stay above the stall speed. As you go higher, temperature also decreases, at least in the troposphere were commercial planes are flying. As the temperature decreases, so does the speed of sound.

Similarly, the critical Mach number is the maximum speed at which the airflow can sustain over the wings without losing lift due to flow separation and shock waves,. Any increase in speed in will cause the airplane to encounter stall effects. When the critical Mach number is exceeded, there is an abrupt rise in drag rise as well as a pitch down due Mach tuck. This can result in aircraft upset, altitude loss and loss of control. As the aircraft descends, the airspeed increases. Excessive pull forces during recovery may lead to further loss of control or structural damage to the airplane.

Modern commercial jet aircraft may suffer both high and low speed stall buffet. The associated boundaries are depicted in the FCOM of the aircraft.  The high speed buffet is caused by flow separation from the wings as occurs behind a shockwave at high altitudes and/or Mach numbers. The low speed buffet is caused by the same airflow separation as the aircraft approaches the stall angle of attack. With stall speed increasing with altitude and sound speed decreasing, the velocity window in which an aircraft can operate becomes narrower and narrower.

Turning manoeuvres at these altitudes increase the angle of attack and results in stability deterioration with  a decrease in control effectiveness. The relationship of stall speeds to critical     Mach number (Mcrit) narrows to a point where sudden increase in angle of attack , roll rates and disturbances cause the limits of the airspeed to be exceeded.

The Coffin corner or the Q corner is the altitude at or near which a high speed fixed wing aircraft’s stall speed is equal to the critical Mach number.  Coffin corner exists in the upper portion of the manoeuvring envelope of an aircraft, for a given gross weight and G – Force.

VMO is an aircraft’s indicated airspeed limit. Exceeding the Vmo may cause aerodynamic flutter and G load limitations to become critical during recovery. Structural design  integrity is also not predictable at airspeeds greater than Vmo.

A deeper understanding of the stall characteristics and recovery procedures are important proficiency issues.  When flying at high altitudes, the crew needs to be aware of the margins of safety available, especially when manoeuvring and while riding out turbulence.

To recover from a stall, the attitude needs to be decreased to reduce the angle of attack. The old maxim of Power for ROD or altitude control and Attitude for airspeed control holds good. A burst of power is not the solution for a stall recovery. In all cases, remember  “attitude before power” when you are in a stall.

WING LOADS

In the aftermath of wing root cracks appearing on A380, lets refresh ourselves on how the wing loading and wing root cracks surface.


Lift produced by a wing is not linear across the wing surface.As a matter of fact,lift is produced by the wing as a result of pressure differential between the top and bottom side of the wing.The pressure differential gives birth to wing shear force and a bending moment, which is the highest where the wing meets the fuselage.

However, aircraft like A330,whose engines are wing mounted, their weight is near the area where maximum lift is being produced.This reduces the total weight, thereby reducing the shear force and the bending moment at the wing root.

Wing loads are also subject to the fuel distribution in the wing.Aiming to achieve lesser moment at the wing root is the objective in effectively managing the wing loading.

OVERWEIGHT LANDING

Definition
All aircraft are certified to land at a particular maximum landing weight. Any landing carried out in excess of that weight is termed as an overweight landing.


Design
Landing at or below overweight landing ensures that normal performance margins as per certification are ensured.
Regulatory certification criteria require that landing gear design be based on 

• A sink rate of 10ft/sec at the maximum landing weight, and 
• A sink rate of 6ft/sec   at the maximum takeoff weight.

Commercial airliners normally make a sink rate of 2-3 ft/sec. A so called hard landing barely exceeds 6ft/sec.

When would it be required to carry out overweight landing?

• In case of any uncontrollable fire, damage, malfunction, etc
• In case of crew incapacitation, medical cases on board requiring immediate attention, etc.
• Any other situation where crew perceive an immediate landing is required.

airbus.com

Is it safe to carry out overweight landing?

Enough debates have been done on this subject. Airline crew are trained to handle overweight landing and the performance criteria and design aspects have been catered to allow for such an event should an emergency arise. Overweight landing provision is limited only to non-normal operation and crew shall not land overweight  on a normal flight  due to direct routings and strong tailwinds.

Aircraft are designed with adequate strength margins for overweight landings.Performance margins are generally well above maximum landing weight. Brakes are designed to withstand reject takeoff at the maximum takeoff weight. So oversight landing should not be a problem.

Is there a special procedure for maintenance after landing overweight?

An overweight landing entails a maintenance procedure even if the landing was smooth!!!!!

Inspections aim at checking for structural distress.

Is there a procedure for crew to follow?

Crew have a procedure to follow which aims at sensitizing crew on the sink rate for touchdown and the technique thereof.

Summary

Overweight landing provision is allowed for by the manufacturer in case of exigencies and the procedure is outlined.Design and performance margins allow for overweight landing.Overweight landing is prohibited in a normal operation. Procedures are outlined for the crew and the maintenance teams  to follow in the event of an overweight landing.

DISTRACTIONS & INTERRUPTIONS IN COCKPIT

Why do they occur??

As per Airbus Briefing notes, the following

are the main reasons identified:

·      Communications

·      Head down activity

·      Abnormal condition or unanticipated situation

When do they occur??

They occur during high workload times, such as during pre-flight preparation, taxi for takeoff, approach  preparation, during briefing prior to descent, descent, during approach, and landing phase.

How can it be addressed??

It can be addressed by robust procedures and policies and adherence to them.

During pre-flight, there is a lot of activity around the cockpit and this is the crucial time that the  interruptions due ground personnel, load and trim sheet, fuelling and techlog  activity, cabin crew coordination, passenger boarding,and so on tend to throw the crew away from the task at hand. Thrust should be on regaining the thread effectively from where it was left and pick it up. Most airline procedures advocate going back to the beginning of the checklist to resume, once the checklist is interrupted.
Crew have made errors in entering wrong load sheet data, due to distraction or interruptions, which in some cases have lead to incidents/accidents.

                     

Also, noted is the case of Eastern L1011 crash at Everglades, which was due to distraction of crew from primary task due to landing gear light bulb malfunction.

What are industry line of defense to counter this??

• ·     Company SOP
• ·     Company policy
• ·     Effective communication
• ·     Sterile cockpit below 10,000ft AAL in flight, and during ground operations
• ·     CRM

EXTERNAL WALK-AROUND

Pic of Brussels airlines

FLIGHT CREW AND CABIN CREW COMMUNICATION

Today's cabin crew are highly trained, highly skilled, and centre on safety as the core of their job function. And, just like the pilots, many have been trained in crew resource management principles. However, some recent findings have uncovered some disturbing facts about the division of responsibilities and safety issues between the cockpit (pilots) and the cabin crew (Cabin crew). The underlying goal for both the pilots and Cabin crew is the safe and efficient completion of a flight. Yet, there has been an unrelenting division of these groups in times of emergencies as well as routine operations. How could this be? As a layperson, you would assume that these groups would be highly cohesive by nature, and yet the opposite has been shown to be true.

The problem with pilot and flight attendant teamwork, particularly in the area of communications, has its roots in the disparate job functions of both groups. When speaking of pilots, it is a mostly male dominated profession. Conversely, when speaking of Cabin crew, it is a mostly female dominated profession. It should be noted, however, that there has been an increased percentage of "gender balancing" over the last few decades for both groups. Theoretically speaking, and this comes from basic innate gender characteristics, male and female thought processes could be somewhat divergent. This is not to say that there is an abundance of testosterone in the cockpit or that females may be influenced by their inherent affective nature, but the gender differences do have to be considered when groups are segregated into mostly male versus mostly female categories.

Besides gender influences, the most salient reason for division in these groups appears to lie in the division of job functions and responsibilities. The cockpit crew is separated from the cabin crew by not only physical barriers (the door), but also communicative barriers (most communication is conducted through an impersonal interphone). Until relatively recently, pilots considered the cockpit "their territory" while the Cabin crew considered the cabin "their territory." Typically, the only times that these two groups would interface was when the pilots needed to be fed, or in the event of an emergency.

"the basic problem is that these two crews represent two distinct and separate cultures, and that this separation serves to inhibit satisfactory teamwork."

Well, we have - the pilots and the Cabin crew have respect amongst one another as friends but when it comes to working as a crew, we don't work as a crew. We work as two crews. You have a front-end crew and a back-end crew, and we are looked upon as serving coffee and lunch and things like that.

By now you have seen the magnitude of the problem; two groups, two cultures, and two completely separate job functions. The pilots, who work in the small but highly complex cockpit—and the Cabin crew, who come from the service-oriented and spacious cabin—having difficulties bringing their environments closer together and working in harmony.

Pilots and Cabin crew need to understand the basic psychology of group dynamics and the positive effect that pre-flight briefings can have between groups. Many pilots and Cabin crew may have never worked together before and yet both of these groups tend to remain isolated before, during, and after a flight. Some captains are better with an introduction and a briefing than others. But overall, there tends to be a "chill in the air" during pre-flight routines.

TAKE OFF PERFORMANCE ERRORS

The Australian Transport Safety Bureau(ATSB) final report of Emirates EK407  tail strike has been released, pointing towards human error in keying of wrong weight and speeds thereof, which resulted in  the aircraft failing to accelerate correctly before over rotating close to the end of the runway and striking the tail several times before becoming airborne and returning to land safely. Although there were no injuries,there was damage to the aircraft and to infrastructure at the Melbourne airport as the aircraft overran in march 2009.The aircraft A340-500, A6-ERG, was damaged as a result. An incorrect weight of 100 tones was keyed in.

Pic from ATSB Report

 In its report the ATSB said: "Previous investigations into similar data entry error and tail-strike occurrences have highlighted the inability of flight crew to conduct a 'rule of thumb' or reasonableness check of speeds when moving between aircraft types.”

Also the report goes on about mixed fleet flying:

"An unintended consequence of mixed fleet flying appears to be a reduction in a flight crew's ability to build a model in long-term memory to facilitate recognition of 'orders of magnitude', or a 'rule of thumb', in respect of take-off performance data."

 The above could have implications on the mixed fleet flying of A320/A330, A330/A340, etc.

Airbus has said that it plans additional development to include functions checking that the aircraft has sufficient runway length to conduct a safe take-off, the ATSB added.

In the history of  aviation, there have been well over 20 such incidents in the past 2 decades or so, involving the human errors in insertion, computation and cross check of data amongst crew  leading to such incidents.

Pic from ATSB report

Will increased automation help in any way to curb the errors of crew in containing this???

A valid viewpoint stems from FAA that such systems, given “all of their inherent complexity”, would be “more problematical than reliance on adequate airmanship”.

What is the way forward in addressing these input  and computation errors and enhance safety??

Airmanship and Procedures

Crew workload is immense during pre-flight when this activity of speed/weight computation takes place. Crew procedures by way of cross checking the data for accuracy and computation, is essential, and coupled with good habit forming and procedural compliance is the way forward.

For crew cross check to be effective, each crew member shall calculate the speeds and thrust settings independently and then cross check to see if there are any differences.

Standard Operating Procedures need to lay emphasis on the same.

Details from loadsheet need to be independently noted by each crew, to avoid any consensus which could lead to an error.

Fortifying crew procedures, cross check of data derived, aircraft systems enhancement  to avoid wrong data insertion and increased thrust in training on this front, will help enhance safety and avoid inevitable tail strikes.

Remember, a wrong keying of numbers on your calculator, will not lead to safety implications, as opposed to wrong keying of numbers on the flight management system.

Procedural  Cross Check  and Compliance, Effective Training and Pre-flight Time Management are the need of the hour.

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