Tag Archives: Aerodynamics

More on AF 447 LOC: Stability, Vg/Vn Diagram and Recovery

There is another very important aerodynamic engineering issue that needs to be discussed when Loss of Control is the subject, and that is Stability, as it relates to high angle of attack flight. Transport category aircraft may not have the same “forgiving” stability as training aircraft, when it comes to bringing the aircraft back into the flight envelope, in the event that Loss of Control has taken the aircraft outside of the flight envelope.
Training aircraft are designed for students, so that they are designed to be easily recoverable from high angle of attack flight and out of controlled flight and flight outside of the flight envelope. The center of gravity, the aerodynamic center, the moment arms of the wings and fuselage, the control surfaces size, types, deploy-ability and locations, the aerodynamic and geometric wing twist, the wings’ angle of incidence on the fuselage and additional itens such as stall strips or other early stall initiation devices, are all assembled so as to make recovery from stalls, post stall gyrations and spins possible for students. This however is not how transport category aircraft are designed and assembled.

Rather, transport aircraft are commercial competitive aircraft, and as such, are optimized for L/D max  (lift/drag max) in cruise, that is the best lift for the cost of drag, for high coefficient of lift devices for take off and landing performance and for relatively good stability (In my opinion, slots, slats and similar devices are incorrectly called high-lift devices. Why? Because you really just want to get the same lift but at lower angles of attack.  They might be more properly be called high “coefficient of lift devices” where the coefficient of lift is maximized by camber and air channels, where the angle of attack is lowered).

But with long wings and long fuselages, when there are very long moment arms with large weights such as passenger/cargo payloads and massive engines, transport aircraft may not perform in the same docile manner as trainer aircraft, acrobatic aircraft and military tactical jets. For line crew members to expect transport aircraft to be as nimble and recoverable as trainers would not be realistic.

Stability is more challenging to engineer when the moment arms and weights get larger.  Recovery from the edge of the flight envelope, that is the Vg/Vn diagram or the Velocity vs G Loading diagram, is possible and quite satisfactory with most transport category aircraft.

But when a flight’s excursion goes well outside of the flight envelope, of the Vg/Vn diagram, when the transition is larger and the aircraft passes well through and past the edge of the envelope, then recovery back into the flight envelop, may not be as easy.

It may still be possible, but testing during development may not have been investigated or engineered. Recovery from outside the envelope may require much more aerodynamic knowledge than a pilot that is qualified just by the requirements of training by regulation may posses.

And if crew members do not possess that knowledge, then there is a problem, because the plane is outside the advertised limits of operation and they may be at a loss on what procedures to use next. So, now what?

Early morning sun rising through clouds.

Early morning sun rising through clouds.

In my opinion, when flight crew members get into the seat of a transport category aircraft, an aircraft  with dozens of people seated behind or into the captain’s seat of a jumbo transport category aircraft with hundreds of people seated behind, flight crew members should be prepared to focus 110% of their attention for the next eight to twelve hours on nothing but flying. Forget about casual conversation. Forget about big meals, you will not starve in 8-12 hours, a sandwich and coffee will do. Forget about all sorts of distractions. Flight crew members need to be focused on keeping the aircraft in the flight envelope by procedures and purely by procedures.

Flight crew members have to know the Vg.Vn diagram and know how to stay well inside of the envelop. Why? That reason is because it may be difficult, very challenging and for some and in some cases, impossible to get back into the envelope once having left the envelope! Is that what happened to Air France 447? I just don’t know, but many of the clues seem to point in that direction.

Flight crew members need to definitely keep the aircraft out of thunderstorms and out of harm’s way for any threat to control and stability. The commercial transport plane is not a trainer, it is not a jet fighter and it is not an acrobatic plane and it is most certainly not a test and development aircraft. It is a massive piece of machinery that has been designed to do one thing extremely well and that is carry large payloads over long distances at exceptionally efficient fuel costs. Attempting to do anything other than that with the aircraft just does not seem “like a good idea.”

As far as members of various boards of investigations are concerned, if board members do not know the aerodynamic principles and physics of large transport plane flight, then it would behoove the board to bring in people who do know these things, such as pilots. Investigations that do not cover this subject area, when part of the flight, especially the fatal last few minutes, occurs outside of the VgVn diagram, are incomplete and therefore as of yet,  little safety value.

These investigations in my opinion are not for the purpose of assigning blame and forming the basis for further legal actions such as dismissal, monetary damages and fines. Lawyers and courts do that kind of work and the case history for lawyer based investigations and court awarded compensation in the Western world goes back hundreds of years.

Instead, these safety investigations are supposed to be about what went wrong and how do we keep this from happening again. They should be about safety.

Whatever the SOP, whatever the aircraft hand-book, whatever the company training program, something was not right in this case and it was up to the board to find that out. But did they find out what was wrong and did they inform everyone else who conduct flight ops with this equipment, on these routes, with these aircraft, how to do so safely? Or did they leave more questions unanswered? Why is the interest in this subject still so high?

How many times have we been given reports of aircraft mishap board investigations stating that something went wrong, and then that they advise someone else, such as the operator, the regulator or the manufacturer to figure it out?

This is incorrect safety investigation theory and procedure. In my opinion it is the duty of the board to find out what went wrong first and then to recommend actions to correct that situation, those SOPs, the aircraft handbook and the training process.

So, to proceed, would not the board need to understand “what went wrong?”

To understand “what went wrong”, investigators would have to know the aerodynamics of high angle of attack flight, the subject of aerodynamic stability, the idea of the Vg Vn diagram  and flight envelop and the entire concept of controlled flight for jumbo transport aircraft.  They also would have to reaffirm that all flight crew members need to know this information through training with their report of the mishap.

Again, JMHO. What do you think? Hey, we are pilots, we have to know this stuff. Shouldn’t the investigators be required to know this stuff as well? How else can they unravel the mystery of a mishap investigation? How else can we move forward to make commercial aviation as safe as possible? How else can we climb into the captain’s seat and take on the responsibility of being a commercial airline captain?100_0306

AF 447: High Altitude Stall or Swept Wing Stall? Did the Mishap Investigation Boards Make a Fundamental Aerodynamic Error?

The mishap investigation boards have given a less than
aerodynamically correct presentation of “high altitude stalls” in the 2009 Loss of Control LOC mishap investigation of AF 447 and the  2005 Loss of Control LOC mishap in Venezuela of a West Caribbean Colombian MD82.
The result is that these mishap investigation reports are not putting out satisfactory recommended corrective actions.
Here’s the problem: Swept winged aerodynamics differs from straight winged aerodynamics. Swept wings stall at the tips first. Straight wings stall at the root first. Swept wings pitch up when they stall. Straight wings do not. Swept wings tend to go deeper into the stall. Straight wings do not.

Pilots of swept winged transport category acft need to know this because many of these pilots received their basic and primary instruction in straight winged trainer
acft, and as such have learned straight winged stall recovery procedures.
Pilots who are now operating swept winged acft, who have not had
specific swept winged stall recovery procedures training per se, may not be adequately trained to handle a swept wing stall.
The various LOC mishap reports of the Colombian MD82 and AF 447 make reference to a problem the boards call “high altitude stall.”  This is really just an explanation that at high altitudes, the spread between true airspeed and indicated airspeed causes longitudinal pitch changes to feel exaggerated. But that extra pitch feel exaggeration is not the problem that keeps the acft stalled from loss of control at FL 370 to impact. What keeps the acft stalled is high angle of attack. The stall is the result of longitudinal controls being in the what is known as the region of reverse control, where the acft is on the back side of the power curve. This means that drag produced by the production of lift, induced drag, is so large, that it is greater in magnitude than the thrust available from the engines, meaning that even at full throttle, the pilots must push the nose down, in order for gravity to add force to thrust, to accelerate the acft forward to a speed great enough, and an angle of attack low enough, to bring the aerodynamic force vector vertical enough so that induced drag is lowered enough so that thrust can now accelerate the acft back into the normal one g flight envelop.

I apologize to all for the very long statement above, especially if it is confusing. In layman’s terms, the pilot has to push forward to get the plane going fast enough to begin flying again.

The boards should have cited swept winged stall and failure to employ swept wing stall recovery procedures as the mishap cause. Recovery procedures for swept wing stalls are different from procedures from recovery procedures for straight wing stalls.
Notice that when both AF447 and the Colombian MD82 descended through lower altitudes, they remained stalled and did not recover. The stall was not a result of high altitude, but high Angle of Attack. In both cases, if the correct swept winged stall recovery procedures had been used, the pilots could have recovered the acft, would have recovered the acft at much higher altitudes than the terrain CFIT and would have recovered the acft immediately, in my opinion. Stall recovery comes by lowering the angle of attack, not by lowering the acft altitude.

This is very important information and needs to be put out.

When a swept wing stalls, the stall emanates at the trailing edge of the wingtip due to span wise flow thickening the boundary layer. The aileron is actually one of the first wing components affected by a swept wing stall. As the stall progresses back up the wing, the aerodynamic center (AC) shifts forward, raising the nose and angle of attack (AoA). This causes the angle of the aerodynamic force (AF) to shift aft, resulting in a rapid and high rise in induced drag (Di), the horizontal vector component of AF. This induced drag opposes thrust, slowing the acft further and raising the AoA, deepening the stall.  This is known as the Region of Reverse Command or the back side of the power curve. Because induced drag rises and rises quickly, there may not be enough power alone to thrust the acft to a higher speed.

The stall is AoA dependent, not altitude dependent. The thrust available is limited by altitude, therefore the thrust deficit above induced drag is altitude dependent. Therefore the only recovery possible is to dump the nose down, reduce the AoA, reduce Di low enough, to where available thrust, as it is added, is sufficient to overcome Di and parasite drag Dp and accelerate the acft Indicated Air Speed (IAS) fast enough to regain lift and therefore one g level flight.

This is the only recovery possible. Swept wing aero is so important to know, that the US Navy has an entirely separate course on it, and it is taught after a flight student has learned to fly straight winged aerodynamic acft. In straight wing aero, the stall begins at the wing root instead of the tip. The AC as a per centage of the mean aerodynamic chord does not shift much, thus AF doesn’t shift aft and doesn’t result in a rapid rise in Di, slowing the acft further. Recovery is quicker with lowering the AoA, adding power and quickly regaining IAS.
Is it possible that the crews of neither of these LOC mishap acft received training in swept wing aerodynamics and the stalls that occur to swept wings?

There is a lot to know in swept wing aerodynamics that is different from straight wing aero, quite a bit to learn (I’ve only touched on it here). This knowledge is critical to understanding swept winged stall recovery procedures and successfully implementing them.

See “Aerodynamics for Naval Aviators,” by H. H Hurt Jr, NAVAIR 00-80T-80, Jan 1965, Naval Air Systems Command,  page 353-354, concerning the “Region of Reversed Command.”

http://www.faa.gov/library/manuals/aviation/

Pilots have got to know this swept-winged aerodynamics if they are going to fly swept winged aircraft safely in all situations, I believe.

In my opinion, this is especially true, if they chose to become test pilots by conducting uncertified operations into FL 600 thunderstorms or operating acft over the certified gross weights indicated for altitude.

Let me know what your thoughts are. Thanx, Paul