On Oct. 14, 2004, a Pinnacle Airlines CRJ on a nighttime ferry flight from Little Rock, Arkansas, to Minneapolis crashed into a residential area in Jefferson City, Missouri. The two pilots, the only people aboard the jet, were killed. The NTSB determined that the accident’s probable cause lay with the pilots, who had exhibited unprofessional behavior, poor airmanship and deviated from standard operating procedures causing both engines to flame out. A part of the fault, it said, was their inadequate training.

That report raised concerns about deficiencies in airman knowledge and training regarding high-altitude/high-Mach operations. In its recommendations the Safety Board said that pilots should possess a thorough understanding of their airplane’s performance capabilities, limitations and high-altitude aerodynamics.

Four years later, the Industry Airplane Upset Recovery Training Aid Team issued a supplement, High Altitude Operations, that noted, “While aerodynamic principles and certain hazards apply at all altitudes, they become particularly significant with respect to loss of control (or upset) at altitudes above FL 250.”

FAA Advisory Circular 61-107A, Operations of Aircraft at Altitudes Above 25,000 ft. MSL and/or Mach Numbers (Mmo) Greater Than 0.75, states, “In recent years, a number of corporate jet airplanes have been involved in catastrophic loss of control during high-altitude/high-speed flight. A significant causal factor in these accidents may well have been a lack of knowledge by the pilot regarding critical aspects of high-altitude Mach flight.”

It goes on to urge pilots unfamiliar with high-altitude and high-speed operations to receive “comprehensive training and a checkout in complex high-performance aircraft before engaging in extensive high-speed flight in such aircraft, particularly at high altitudes. The training should enable the pilot to become thoroughly familiar with aircraft performance charts and aircraft systems and procedures. The more critical elements of high-altitude flight planning and operations should also be reviewed.”

And during the checkout, it says, the pilot should “demonstrate a comprehensive knowledge of the aircraft performance charts, systems, emergency procedures and operating limitations, along with a high degree of proficiency in performing all flight maneuvers and inflight emergency procedures.” Only then can the pilot operate safely in such an extreme environment.

With the forgoing in mind, have adequate changes been made in the industry to correct these systemic deficiencies? To be more specific, in your most recent training, did you get ground school and then practical hands-on experience recognizing and recovering from the onset of low-speed and high-speed buffet, airspeed decay, malfunction of a flight control system necessary for high-Mach/high-altitude flight such as a Mach trim or yaw damper system, or recovery from an autopilot malfunction? Probably not.

The industry team that assembled the High-Altitude Operations training supplement designed a line-oriented flight training (LOFT) scenario for use in a simulator that highlights many of these important topics (with the caveat that the simulator has the ability to realistically replicate the aircraft’s performance and handling at high altitude.) The scenario begins in a near-maximum altitude situation at a weight that would be at or near the maximum for that altitude.

Over the years, crews have caused their airspeed to degrade at high altitude when trying to top thunderstorms, when deicing equipment performed inadequately, because they were unfamiliar with high-altitude performance, turbulence, mountain wave penetration and when they failed to recognize atmospheric changes. Slow climb and cruise speeds, often caused by using inappropriate automation modes, also have caused problems. When flight crews climb at an increased rate, sometimes necessary due to traffic or weather concerns, or when misusing the vertical speed mode, they can inadvertently put the aircraft below L/D max. According to the authors of the special report, inappropriate use of vertical speed modes has been involved in the majority of slow-speed events during high-altitude climbs.

Flight slower than L/D max must be avoided in the high-altitude environment since within that so-called “region of reverse command,” any increase in drag will cause a further deterioration of airspeed, which in turn causes more induced drag. Without corrective action, the aircraft will continue to slow and eventually stall. To prevent further airspeed deterioration the pilot must reduce bank angle and increase thrust to maximum continuous. If these actions are insufficient, it will be necessary to trade altitude for airspeed to get out of the slow flight regime. Due to an aircraft’s narrow flight margins at altitude as well as the decreased thrust available, flight crews have less time to recognize and respond to airspeed deterioration.

It is important for pilots to be familiar with the use of charts showing cruise maneuvering and buffet limits. A classroom is a good venue to begin understanding these important concepts, but it is vitally important that a pilot knows how to apply this knowledge. A full flight simulator provides an excellent learning environment for this step, all of which makes this LOFT scenario particularly valuable in an airman’s development.

FMS entries are particularly important in order to get accurate information on the PFD such as the amber airspeed warning bands. These entries include correct aircraft weight as well as an accurate temperature deviation at the cruising altitude. 

More sophisticated FMSes may allow a pilot to insert entries for maneuver margin. A maneuvering margin of 1.3 g is commonly used in the airline industry. The training aid suggests discussing buffet margins at this point, which would include referencing the airspeed warning bands on the PFD. It is vital to realize these airspeed bands do not give any indication of thrust limits, and a primary factor limiting thrust production at altitude is warmer-than-standard temperatures.

Cessna’s High-Altitude Aerodynamics explains that “by flying at a high altitude, the pilot is able to operate at flight levels where fuel economy is best and with the most advantageous cruise speed. For efficiency, jet aircraft are typically operated at high altitudes where cruise is usually very close to rpm or exhaust gas temperature limits. At high altitudes, little excess thrust may be available for maneuvering.” This means an aircraft will be sluggish to accelerate because the added thrust is so marginal at altitude. Meanwhile, the High-Altitude Operations training supplement emphasizes that most jet transport airplanes are thrust limited, rather than low-speed buffet limited, at altitude. This is especially the case in a turn.

Analysis of NASA’s Aviation Safety Reporting System (ASRS) reports by this author for a scientific paper entitled “Investigating and Preventing the Loss of Control Accident,” presented to the International Society of Air Safety Investigators (ISASI) Conference in September 2011, revealed numerous instances in which flight crews failed to notice that the static air temperature had warmed significantly after they flew through a weather front. Since engines produce even less thrust in warmer air, these events resulted in unanticipated airspeed decay, which required the pilots to take corrective action. It’s smart to keep an eye on the static air temperature, knowing that warmer-than-standard temperatures are going to lessen thrust output.

The training aid suggests using this scenario to show how changing ambient conditions induce airspeed decay. Another method for demonstrating that decay is to enter into a 30-deg. bank turn. The extra drag induced by rolling into a 15-deg. bank is modest, but the drag increase at double that bank is considerable. Several of the ASRS reports noted earlier indicated flight crews were making turns to avoid weather and were surprised at the airspeed decay resulting in low-speed buffet, or that rolling out of the turn didn’t resolve the problem because the airspeed loss in the turn put the aircraft on the back side of the power curve.

Using the wrong automation mode when maneuvering at high altitudes can result in a deterioration of the aircraft’s safety margins. To avoid adding too much drag on the aircraft when turning at high altitude, use LNAV to ensure bank angle is limited to respect buffet and thrust margins. Conversely, the use of HDG or hand flying does not provide bank angle protection and can result in unacceptable deceleration of the aircraft and buffeting. Thus, if using HDG mode, it’s advisable to use the half-bank feature to limit the bank angle.

During this simulator exercise, once the aircraft has begun to decelerate it is important to note the early indications such as airspeed trends, trim changes, low airspeed indications, and so forth. Since autopilot is normally used in actual high-altitude flight operations, it is recommended that it be left on, if possible.

An important objective of this exercise is for the flight crew to recognize the early onset of airspeed deterioration and recover correctly. The pilots will be able to see the progression of the visual and aural alerts, as well as stick shaker, stick pusher action and feel the buffeting that’s characteristic of that aircraft. Simulator fidelity may be questionable if the aircraft is allowed to deteriorate into a stall or upset. Reducing bank angle, commanding maximum thrust, and a smooth reduction in pitch to lower the angle of attack (AOA) and allow the aircraft to accelerate are necessary steps in the recovery.

On many business jets slow speed buffet tends to be the first stall identifier in the high-altitude environment. A stall can also be characterized by a lack of pitch authority, a lack of roll control and/or an inability to arrest the rate of
descent.

At higher altitudes there is insufficient excess thrust to “power out” from a stall while attempting to minimize altitude loss. It is impossible to recover from a stalled condition without reducing the AOA deliberately and smoothly and trading altitude for increased airspeed.

A deliberate design feature of the simulator training scenario is that given the aircraft’s weight and near-maximum altitude, it will be necessary to begin a descent. A descent of at least 1,000 fpm is recommended. Out in the real world it would be vital for the flight crew to monitor TCAS, scan for traffic conflicts and notify ATC immediately.

Excursions beyond Mmo similarly have adverse consequences on an aircraft’s safety. Years ago, there were some notorious cases in which early-generation business jets had their airspeed and Mach warning systems deliberately disabled to permit intentional excursions beyond their FAA-certificated Vmo/Mmo limits. These actions led to several fatal accidents. Fortunately, this type of accident has virtually disappeared.

The ISASI study did uncover a number of ASRS reports revealing unintentional excursions to the high end of the speed envelope. The results for such vary by aircraft design but in general involve aerodynamic flutter, aileron buzz or snatch, high-speed buffet, loss of control surface authority and Mach tuck. Any of these can critically jeopardize the safety of the aircraft and those aboard.

Many business jets are designed with some form of trim and autopilot Mach compensating device, such as a stick puller, to alert the pilot to inadvertent excursions beyond its certificated Mmo. This stick puller should never be disabled during normal flight operations in the aircraft. It is important to know the MEL restrictions when these devices are inoperative, as well as the proper inflight corrections (to include the changes to an aircraft’s handling characteristics) if they fail.

Encountering mountain waves at high altitude creates a “triple threat” of turbulence, low-speed buffet and high-speed buffet. Gust loads created by mountain waves can increase the local Mach speed over the wing, resulting in shock-induced buffet or even a stall.

According to Cessna’s High-Altitude Aerodynamics supplement to the Citation Excel training manual, “Increasing either gross weight or load-factor (g-factor) will increase the low-speed buffet and decrease Mach buffet speeds. A typical turbojet airplane flying at 51,000 ft. altitude at 1.0 g may encounter Mach buffet slightly above the airplane’s Mmo (Mach 0.82) and low-speed buffet at Mach 0.60. However, only 1.4 g (an increase of only 0.4 g) may bring on buffet at the optimum speed of Mach 0.73 and any change in airspeed, bank angle or gust loading may reduce this straight and level flight 1.4 g protection to no
protection.”

In addition, updrafts and downdrafts within these high-altitude waves cause significant handling difficulties, which can lead to loss of control. According to the FAA’s Airplane Upset Recovery Training Aid, an aircraft attempting to maintain a level altitude on autopilot in the updrafts and downdrafts of a wave will experience significant changes in pitch and airspeed.

In the downdraft sections of the wave, the aircraft’s autopilot will pitch up to maintain altitude. Due to the general lack of excess thrust to maintain a constant airspeed (at FL 410, a typical business jet engine produces only a quarter of its takeoff rated thrust), the airspeed will decay.

When the aircraft flies into the updraft portion of the wave, the nose of the aircraft will pitch down to maintain altitude. In relatively sleek jets the airspeed will quickly accelerate, possibly rapidly exceeding Vmo/Mmo and exhibiting undesirable aircraft handling characteristics. Remember that as the freestream Mach number increases, so does the Mach number over the wing, creating stronger shock-induced separation of the boundary layer, ultimately resulting in an even lower critical AOA. ASRS reports indicate clear confusion at the alternating onset of low-speed and high-speed buffet with pilots unable to accurately diagnose between the two.

Pilots flying at high altitudes in areas where turbulence and/or mountain waves may be expected must carefully consider what acceptable safety margins will be necessary to accommodate the sudden and unexpected vertical accelerations that they might encounter with little or no warning.

Each aircraft design will exhibit its own unique indications when nearing the limits of the flight envelope. Unfortunately, these distinct warning signs may not be revealed in a flight simulator. As discussed in “Upset Recovery in Simulators” (B&CA, April 2012, page 34), aerodynamic complexities grow in magnitude as the limits of the aircraft’s envelopes are approached. There is no assurance that the math models in the simulator’s software will accurately replicate the aircraft’s actual behavior near the edges of the flight envelopes.

Others might be tempted to suggest using actual aircraft to get hands-on training to learn the indications of approaching the aircraft’s flight envelope limits, but that’s for test pilots, not trainers. According to Cessna’s High-Altitude Aerodynamics, “Jet aircraft operating at high altitudes and high Mach numbers may simultaneously experience problems associated with slow-speed flight such as Dutch roll, adverse yaw and stall. In addition, the reduced air density reduces aerodynamic damping, overall stability and control of the aircraft in flight.” An abundance of accidents during training provide sad evidence that aircraft can be unforgiving due to limited margins.

Fly-by-wire (FBW) aircraft have design features that protect them from exceeding design limitations, but these have caused handling problems during a high-altitude mountain wave encounter. For instance, during overspeeds some aircraft have automatically pitched up to avoid any further overspeed and in the doing deviated significantly from the assigned altitude. In some instances, flight crews have been unable to re-assert direct control over the aircraft until the automatic safety device relinquishes its control. Obviously, this is a situation that a flight crew should not encounter for the first time in actual flight, but rather in a simulator.

What is the key information that a pilot must understand well and be able to apply in high-Mach flight? The NTSB’s recommendations provide a good starting point for gaining a thorough working knowledge to safely operate an aircraft in this unforgiving region of the flight envelope. A pilot should understand which flight instruments are reliable or not in this environment. High-altitude, high-Mach flight is often conducted with the aid of yaw dampers and Mach trim. When these malfunction a pilot must know the proper recovery procedures. Trust me on this, you don’t want to discover your aircraft’s Dutch roll mode with an inoperative yaw damper while on a trip if you’ve not been trained.

The interaction of separated airflow and Mach waves on the wings can excite abnormal vibrations in flight controls if they’ve not been properly maintained. A pilot needs to know the correct preflight check of flight controls, especially after the aircraft’s been in maintenance, and to recognize the early inflight warning symptoms if the flight controls are improperly rigged.

Further, FMS entries have a direct bearing on the information presented on the PFD and thus need to be accurate. Improper mode usage of the autopilot can cause the aircraft to enter into undesired aircraft states. Proper automation usage and appropriate manual control inputs are imperative. And lastly, high-altitude atmospherics such as mountain waves, jet stream dynamics and high-altitude turbulence must be well understood for pilots to make the right decisions regarding flight-path altitude and routing to avoid conditions in which the aircraft’s safety margins would be quickly degraded.

NBAA Safety Committee Chairman Steve Charbonneau believes that “up until now, nobody has really put together a safety program that specifically addresses the skill sets required of business aviation pilots.” And the committee has expressed concern that “today’s recurrent training process basically recertifies pilots rather than teaching them new skills or sharpening old ones.”

All pilots need access to training that has proven to be effective. And no circumstances demand more effective training than when transitioning to go high and fast. Placing oneself near the edges of a performance envelope demands special knowledge, skill and vigilance to do so with confidence and safety. Acquiring that education benefits all, including you. B&CA

View “Upset Recovery Supplement” and “B737 High Altitude Maneuvering” or go to AviationWeek.com/Upset_Recovery and AviationWeek.com/B737_High_Altitude