Mentor Matters: Bugging target airspeed

Add to the large range of operational speeds the inherent slipperiness of jets, and it’s not surprising that improper energy management can be linked to so many accidents. Both in accidents caused by excessive energy, typically presenting as landing phase runway excursions, and in those caused by inadequate energy—stalls and other loss-of-control events—pilots too often are not up to the task of maintaining the aircraft’s energy in what can, at times, be a quite narrow optimal band.

The total energy present in an aircraft at any time can be thought of as distributed between two subtypes: the potential energy, present at any height above zero feet agl, and the kinetic energy, caused by its movement. Think of an aircraft flying an ILS, either being above the glideslope (excessive potential energy) or over target V_REF (excessive kinetic energy), as having an undesirably high energy state that could lead to challenges stopping the aircraft. In the air, jet aircraft have only one strategy to shed excessive energy: wait for what drag is present to dissipate the energy.

The pilot can, of course, do things to increase drag: deploy landing gear, flaps, or speed brakes, or fly airspeeds slower than minimum drag speed, but ultimately there is a limit to how rapidly energy can be dissipated. This makes forward thinking of utmost importance. Remembering that time is the final arbiter of energy reduction, a pilot is always well-served by initiating an energy-change strategy sooner rather than later. If you find yourself on a 10-mile final at a reasonable altitude but a touch slower than anticipated, it’s easy to bump up the power to add energy, but it’s far more difficult to shed energy if your planning is late and your technique poor.

Remembering that time is the final arbiter of energy reduction, a pilot is always well-served by initiating an energy-change strategy sooner rather than later.Many pilots hinder their efforts to shed energy with suboptimal thrust management. Barring environmental variables, such as thermals or ridge lift, the only way a pilot can add energy to the aircraft is through the conversion of stored energy (in the engines’ fuel-air combustion chamber) into thrust. From the proverbial armchair perspective, it’s apparent that any thrust setting above idle is counterproductive as an effort to lose energy. Frequently I see task-saturated pilots push the nose down when they realize they are high on an approach—but they only make a small power reduction. Unless the energy to be shed is very low (e.g. 10 to 20 knots above V_REF, or a half dot high on the glidepath), idle power is more often the correct action.

Modern FADEC-controlled jet engines spool up rapidly, so there’s no danger in going to idle thrust, provided the aircraft is at an appropriate altitude. What defines an appropriate altitude should be well-known by now, given decades of training emphasis on stabilized approaches. The criteria for a stabilized approach emphasize: 1) being configured for the approach (landing gear extended and flaps at the correct setting); 2) flying within 20 knots of target airspeed; 3) staying within one dot’s worth of needle deflection from the CDI’s central on-course, on-glidepath indication; and 4) no warning flags. In instrument meteorological conditions, those criteria must be met no lower than 1,000 feet above field elevation, or no lower than 500 feet above field elevation in visual conditions. Ignore those guidelines—for example, being late to configure, or slamming power to idle in an attempt to slow down—and you’ll be playing a game of catch-up with energy management. Very often, the only way to save the approach is to fly a missed approach or go around and try again.

While runway excursions caused by an excessive energy state may be more numerous than accidents triggered by low energy, the outcome of in-flight stalls and loss-of-control events tend to be far grimmer, often involving multiple fatalities. A stall can, of course, occur at any airspeed given sufficient loading on the aircraft, but many events occur in 1-G unaccelerated flight, and are simply attributable to inadequate airspeed management.

Given the lack of aural cues from wind noise, it’s critically important that jet pilots regularly scan their airspeed during climbs, descents, and other phases of flight when making power and altitude changes. A subtle, but critically important corollary to this practice is that the pilot must always have in mind a target and minimum safe airspeed for the airplane’s configuration and weight. Some manufacturers provide recommendations in the form of absolute minimum speeds for specified flap settings, while others recommend that additional airspeed factors be added to calculated V_REF values under certain conditions. For example, V_REF plus 30 knots for no flaps extended, and V_REF plus 20 knots for partial flap settings.

For jets with pneumatic leading-edge deice boots for icing protection, a further consideration must be respected: an increase in minimum safe speed when any amount of residual ice is present. Manufacturers typically mandate both a minimum speed in icing for all flight phases except approach, plus an increased V_REF if the wings cannot be verified to be completely clear of ice accretions. Two notable recent accidents were attributed to aerodynamic stalls occurring at higher than normal speeds because of ice contamination. Had the pilots been operating the deice equipment properly, the stall warning system would have raised the threshold at which the warning would have occurred, giving the pilots a chance to correct the dangerous state before it became too late.

However minimum airspeeds are calculated, the pilot needs to be constantly aware of where the current speed is relative to this minimum. A good way to boost situational awareness is to bug the minimum airspeed. The brain interprets visual symbols far quicker than it processes raw numbers. So, if you need to fly at 150 knots and bug that airspeed on your primary flight display, you’ll see if you’re below bug speed much sooner than if you had set only the “150” numerals in a smallish airspeed selector window. That’s especially true when a high workload demands a rapid instrument scan.

Depending on the avionics manufacturer, there may be a dedicated knob for setting a reference airspeed bug regardless of current autopilot mode or, less optimally in some airplanes, there may be a menu option to bug an airspeed. Whatever method you use, it’s worth your time to develop the habit and the “muscle memory” to update the bug throughout a flight, as target speeds change. AOPA

Neil Singer is a master CFI with more than 9,500 hours.