Factors To Consider When Transitioning From Props To Jets, Part 2

Jet aircraft exhibit a wide spectrum of behaviors in high-altitude flight that are not adequately taught in ground schools and usually given scant attention in simulator training.

This begins with the changes in an aircraft’s handling characteristics at high altitude. The vertical tail becomes less sensitive to sideslip angles, which decreases directional stability. The horizontal tail contributes less damping to longitudinal pitch oscillations, thus the jet becomes more responsive to control inputs. Do not overreact with large and drastic inputs.

Dutch roll is the tendency of the aircraft to roll when it is yawed. Yaw dampers effectively counter the Dutch Roll mode in jets. If a yaw damper is inoperative, untimely inputs of the rudder will greatly aggravate the recovery. Since the Dutch Roll becomes more exacerbated with higher altitudes and/or higher speeds, the recovery tactic likely prescribed in your aircraft manual in case of a loss of yaw damper is to descend and slow down.

A lesser recognized characteristic of swept wings is the steep rise of induced drag on the back side of the power-required curve. This leads to deteriorating airspeed which can degrade into a loss of control. If a pilot does not recognize the airspeed deterioration early in the process the recovery options will be hindered due to the marginal thrust available. Recovery requires reducing the angle of attack, and practically speaking, this requires trading altitude to gain airspeed to get back on the front side of the power curve. The altitude loss can be considerable.  

Adhering to the recommended climb speed is important to prevent this. Under no circumstances should airspeed be allowed to fall below the target figure since it increases AOA (drag) and fuel consumption and requires a high power setting to accelerate to the cruise speed. Once again, simulator training doesn’t expose trainees to the conditions that can create this potentially dangerous flight control.  Heavier weights, using the wrong autopilot mode to make turns at altitude, and warmer than ISA temperatures have been contributing factors in past incidents.

Mach waves formed over the wing cause an oscillating cycle termed buffet. Every airfoil has its high speed and low speed limits (buffet margins), and these margins will vary by the altitude. For example, flight testing on a common business jet found that the AOA for buffet onset at low altitude was 17 deg., whereas the buffet onset was at 8.5 deg. AOA when cruising at FL 340 at a speed of Mach 0.55.  Furthermore, the buffet onset occurred at 7 deg. AOA when cruising at FL 450 at a speed Mach 0.65. The buffet onset AOA varies inversely with the Mach number, meaning that at faster Mach Numbers, the onset of buffet occurs at lower AOAs.

The buffet margins set a boundary between a safe flight region and a part of the flight envelope in which the jet may encounter serious control problems or the structure is significantly affected by potentially dangerous shaking that would cause fatigue loads. As altitude increases, remember the indicated airspeed (IAS) for low-speed buffet increases, whereas the high-speed buffet speed decreases.  The result is that the margin between high speed and low speed buffet decreases.

The amber band on the primary flight display (PFD) will present these buffet margins.  Be advised that the amber band does not give any indication of thrust limits. It is important to understand that accurate flight management system entries are particularly important in order to get accurate information on the PFD. These entries include correct aircraft weight as well as an accurate temperature deviation at the cruising altitude.  

Flight Control Surface Aeroelasticity

Flight controls at higher speeds can be susceptible to aeroelasticity. Shock waves induce separated airflow over flight control surfaces causing the flight control surface to move. This cycle repeats itself at a high frequency and causes a “buzz” in the flight controls. If this buffet is strong and prolonged, it can cause structural damage.  

Flight controls powered by cables and pulleys are more susceptible to this. Worn bushings and loose cables are common contributors. This is yet another example in which jet pilots need to have specific in-depth knowledge about the aircraft’s systems, operating characteristics, limitations, preflight inspections as well as the proper recognition and recovery when a component malfunctions.

The earliest generation of high-performance business jets revealed many shortcomings in pilots’ aerodynamic knowledge and skills. Among these was the Mach tuck.  

According to the U.S. Air Force Test Pilot School’s “Flying Qualities Theory and Flight Test Techniques,” the initial shock waves will form over the wing, beginning near the relatively thick wing root. This can reduce the downwash over the tail, producing a nose-down tendency. More significantly, it causes aft movement of the wing’s center of pressure due to the change in the wing’s center of lift. This results in the nose-down “tuck” tendency that worsened as the aircraft’s speed increased.

By the way, conventional upset recovery techniques made this worse. Extending the spoilers at speeds above Vmo/Mmo caused an additional nose-down pitching moment.  

Gates Learjet Service News Letter 49, dated May 1980, specifically urged careful review of procedures relating to emergency descent, inadvertently exceeding Vmo/Mmo, pitch axis malfunction, and normal or primary pitch trim system runaway for the proper preventive and recovery procedures.  

Descent Management

The low drag design of modern business jets means that descending from altitude and properly managing the energy of the aircraft can be challenging. Traditional simulator courses are focused intently on practicing the instrument departures and approaches, thus trainees do not get exposed to important aspects of descent management.  

What are the “Best Practices” exhibited by professional flight crews to smoothly execute the descent? Get ATIS information as soon as possible during the low workload of cruise flight, then conduct your Landing Performance Assessment.  Load the arrival and anticipated approach into the FMS, then brief the descent and approach, even if you are in a single-pilot operation.  Ideally this should be accomplished at least ten minutes prior to top of descent.

The typical descent profile is based upon losing approximately 3,000 ft. per 10 nm, or what a lot of us have simply called “the 3:1 rule of thumb.”  In other words, if you are cruising at 40,000 ft., you should anticipate getting a descent clearance no later than 120 nm away from your destination. If air traffic control hasn’t given you a descent clearance by then, you need to begin requesting ATC for a lower altitude and realize that you are getting into a situation in which your jet is likely to be too high and too fast as you near your destination. This usually deteriorates into an unstabilized approach.

Beware of the winds aloft. This becomes painfully obvious on eastward-bound descents with 100+ kts. Jetstream winds on the tail. Increase your distance for descent by 2 nm for each 10 kts. on the tail.  Clouds during descent may require use of anti-ice, which would require keeping engines “spooled up.” In this case, a 5:1 ratio may be more fitting for your descent planning.

Throughout the entire flight a “next target” should be defined, to include position, altitude, configuration, speed, vertical speed (if applicable), and power setting. If it becomes anticipated that the aircraft won’t meet one or more elements of the next target, then preemptive corrective action should be taken without delay.

It is prudent to back-up your FMS navigation by referring to “raw data” navaids and instruments, using the approximate rules of thumb.  At 30 nm from touchdown your jet should be no higher than 9,000 ft. AGL.  At 15 nm you should be within 3,000 ft. AGL of the destination.

Flight crews should predict the effects of the runway surface condition in their Landing Performance Assessment, we advise in Part 3 of this article. 

Click here for Factors To Consider When Transitioning From Props To Jets, Part 1.