Flying pitch mode

In addition to lateral path management that flies holds, procedure turns, and full missed approach profiles with a single button push, the autopilots manage vertical control of the aircraft in a sophisticated manner. Whether tracking vertical navigation (VNAV) profiles in climb and descent, with automated transitions in and out of altitude hold as appropriate, or maintaining an airspeed profile that complies with airspace limitations (e.g., 200 KIAS in a terminal area, 250 knots under 10,000 feet) while also optimizing climb efficiency, contemporary autopilots are truly worthy of the moniker “automation” that is informally used to refer to the combination of the flight management system (FMS) and autopilot.

Yet as complex as an autopilot’s modes can be, there are occasions when lower levels of automation are appropriate. For example, when a pilot has any uncertainty about the current programming of the FMS, coupling the autopilot to the FMS via the navigation (NAV) mode is a sure-fire gate to the dreaded “What’s it doing now?” state of confusion. Putting the autopilot into heading mode, in contrast, is unlikely to create any misunderstanding about which way the aircraft is going to point, freeing up mental resources for debugging the FMS.

Use of the oldest and most basic vertical mode is a powerful tool in a pilot’s arsenal, yet one so infrequently used that many pilots I train are either unaware of its existence, or unsure of how to utilize or even select it. I’m referring to pitch (PIT) mode, a mode so rudimentary that many avionics manufactures consider it the default vertical mode, i.e., the mode that exists when nothing else has yet been selected. Despite the “placeholder” label assigned to pitch mode, it is an extraordinarily useful mode—especially for the takeoff and climb phases of flight.

As the name implies, pitch mode simply commands the autopilot to hold a specified pitch attitude. Its first advantage is in its very simplicity, as it can be thought of as a first-order command to the autopilot. If the autopilot is asked for 5 degrees nose up and turbulence disturbs the aircraft from that pitch, the autopilot simply needs to control the elevator to return the nose to 5 degrees. Contrast this with the operation of the autopilot in vertical speed or airspeed hold mode—the autopilot constantly needs to translate a desired state (e.g. 1,000 feet per minute of climb) into an equivalent pitch attitude, and then through trial and error make ongoing adjustments to the target pitch while also maintaining that pitch through the elevator servos.

Since pitch mode, in effect, cuts out the middleman it can be a far smoother mode, notably in turbulence or at high altitude. There is less hunting and over-correcting of the pitch attitude, which often results in a noticeably better ride for passengers. Enter our first compelling use case for pitch: climbing above 30,000 feet.

It is considered a best practice to avoid the use of vertical speed mode during climbs at higher altitudes, as there is an ignoble history of pilots stalling aircraft through unobtainable vertical speed commands. If, because of the ineluctable effects of weight, altitude, and temperature, an aircraft is incapable of maintaining the pilot-selected vertical speed, the autopilot will nevertheless continue to command an increased pitch attitude—until either stall occurs or the pilot intervenes.

For this reason, many pilots have developed a practice of using the Mach hold mode when climbing at higher altitudes. Yet in the thin air above 30,000 feet, it does not take much disturbance for the indicated Mach to vary slightly from commanded Mach. The unending attempts of the autopilot to precisely hold commanded Mach results in the dreaded porpoising—constant, slow oscillations above and below the necessary pitch attitude.

Use of the oldest and most basic vertical mode is a powerful tool in a pilot’s arsenal, yet one so infrequently used that many pilots I train are unaware of its existence.

Climbing in pitch mode, however, small oscillations around desired Mach are accepted for a rock-steady pitch attitude. The ride will be noticeably more comfortable for passengers, and if the aircraft runs out of climb ability it will simply enter level flight in the commanded pitch attitude (typically of three to five degrees nose up at high altitude), even if the pilot is completely inattentive to energy state. To enter pitch mode the pilot simply deselects the current vertical mode—if climbing in vertical speed mode, for instance, pressing the VS button will cause the autopilot to switch to pitch.

The opposite end of our flight envelope presents a second use case for pitch mode—initial takeoff and level-off. As pitch is the default vertical mode, it will be the commanded mode for takeoff when the takeoff/go-around (TOGA) button on the thrust lever is pressed. Instead of being annunciated on the PFD status bar as PIT, for most light jets the vertical mode will present as “TO,” which is simply a predefined pitch attitude of around 10 degrees up; it’s important to keep in mind that TO still is, in every way but name, pitch mode.

Consider a situation when a light jet such as an Embraer Phenom 300 or Cessna Citation CJ4, with excellent climb performance when lightly loaded and/or in low density altitude conditions, is assigned a low initial level-off of 1,500 feet above the departure airport. A common after-takeoff workflow is to switch the flight director/autopilot vertical mode from takeoff to airspeed hold (typically labeled flight level change, or FLCH) after the flaps are retracted at around 400 feet. When the FLCH button is pressed, the autopilot will typically capture airspeed present at the time of button push as the target speed. Our rapidly accelerating aircraft, however, has quickly passed through that speed, and the autopilot responds by pitching up, sometimes dramatically. The pilot is likely simultaneously increasing the commanded speed, but often the aircraft will accelerate faster than the pilot can adjust the target. The result is an uncomfortably high pitch attitude and rate of climb, and in some cases an overshoot of the target altitude, even with the autopilot engaged.

A simpler and calmer workflow is to leave the flight director/autopilot in takeoff mode, and after flap retraction adjust the target pitch downward from the 10-degree TO setting to 5 to 7 degrees. This can be done most easily through the yoke-mounted control wheel steering (CWS) button—a natural complement to use of pitch mode. The pilot simply lowers the nose to the desired attitude then presses and releases the CWS button. The flight director will now be synchronized to the attitude existing when the CWS button was released—no need to let go of the throttles to adjust the autopilot control panel. Also, there’s no rapid pitch up as the accelerating aircraft tries to recapture a commanded airspeed. Combined with a judicious power reduction as the target altitude is approached, this technique results in a dramatically smoother and lower stress altitude capture.

While the first two use cases focused on the practical, I’ll close with a third case that is more focused on the aesthetic. I’ve come to greatly enjoy using pitch mode for climb between the low-altitude initial climb/level-off phase and the higher altitude stall-avoidance phase. Unless equipped with autothrottles, a jet’s energy management during climb is almost always performed at a fixed thrust setting (maximum climb thrust), and thus all the pilot needs to manage is a tradeoff between airspeed and climb rate. Given this, pilots most commonly climb in FLCH speed-hold mode, set to the most efficient cruise climb speed as determined by the manufacturer. With this task done, there’s not much left for the pilot to do when the autopilot is engaged other than monitor the autopilot to ensure it’s performing as expected. I find once FLCH is selected for an extended climb and the autopilot turned on, a mental distance develops slowly between the pilot and the aircraft; as now the automation is controlling the climb, the pilot subtly checks out.

In contrast, I’ve found performing the entire climb from takeoff to cruise altitude in pitch mode forces me to stay in touch with the airplane’s performance in a noticeably more connected manner. I’m far more aware of how the airplane is performing: What speed am I seeing at 6 degrees pitch? What about 5? What climb rate do I see if the airspeed increases 10 knots above the nominal cruise climb speed? Without increasing workload in a meaningful manner, using pitch mode throughout climb keeps the pilot more engaged with and aware of the airplane’s performance and energy state, always a laudable goal.