How Slow Will You Go?

By Mr. Bill Krouse, AMC/A3TO

We have all heard the adage, “The three most useless things to a pilot are altitude above you, runway behind you, and gas still in the fuel truck back home.” Forty years of flying, instructing, and studying aviation mishaps inspires me to add a fourth—“The airspeed you don’t have,” which transitions to the question posited by this article: How slow will you go to facilitate an enroute rendezvous, to meet your time over target, to enable a Container Delivery System (CDS) airdrop, or while holding for your delayed cellmate to rejoin? Preflight briefs will cover speed considerations for departure and arrivals, emergency recoveries, and standard procedures for airdrop and aerial refueling operations; however, since you cannot brief every possible scenario, do you have a general minimum speed you will use when the best-laid plan falls apart?

Military Flight Operations Quality Assurance (MFOQA) analysis shows many pilots may not have considered what speed limits they will use. I am referring to the MFOQA analysis of predominately those Aircraft Mission-Design Series (MDS) with missions that included aerial refueling and low-level operations, such as KC-135 flights cruising more than 40 knots below endurance speed, C-17 crews triggering the aircraft deep stall prevention system below 750 feet, and C-130J crews decelerating to as slow as 104 knots during a CDS airdrop. This article outlines factors affecting high-altitude flight and highlights the extent to which some crews are slowing their speed.

A cruise-related urban legend states, “The slower I go, the more gas I will save.” This expression is only accurate to a point. To understand why it is not a fact, review the principles of drag and the relationship of the drag components. Since the aircraft’s gross weight is relatively stable over short periods of time, the principal component of lift that a crew has ready control over is total drag. Total drag is the sum of the induced drag, directly related to lift productions, and parasitic drag, all drag not associated with the production of lift (that is, drag caused by the aircraft form, airflow interference by specific components, and skin friction) at a given airspeed (Figure 1).

Figure 1 shows that as airspeed increases, induced drag decreases. This reduction occurs because the aircraft’s angle of attack decreases as airspeed increases. Inversely, however, parasitic drag increases in proportion to the square of the aircraft speed. Therefore, when the induced drag and parasitic drag curves intersect, it creates the minimum total drag—sometimes called Lift/Drag Max (L/D Max). At L/D Max, the aircraft will operate at the most energy-efficient speed, and traveling either faster or slower will cost more fuel. Thus, going any slower than minimum drag speed will not save fuel. More importantly, speed less than L/D Max will increase the angle of attack required to maintain level flight. As the speed slows, the angle of attack will increase to the point where there is insufficient thrust available to maintain level flight, or the wing exceeds the stall angle for producing lift—the result is the same: the aircraft will stall. When the pilot has a “hip-pocket” minimum speed that they will not exceed, it prevents them from having to complete mental gymnastics during high-stress situations. How often are these slow events occurring? For this article, the MDS analysts expanded their search beyond the considerable amount of analyses already completed on approaches and looked at flight data outside of approach criteria.

The C-17 analysts focused on the Alpha Limiting System (ALS), which is designed to prevent a deep stall. In the 12-month cycle ending October 2019, there were 176 ALS-activation events captured in the flight data. Of those activations, 16 were above 10,000 feet, and 74 were below 750 feet above ground level, with many activating during aggressive maneuvering while conducting low-level operations. Furthermore, of those 176 ALS-activation events not related to aggressive low-level maneuvering, peeling back the analysis onion an additional layer showed that some of those activations occurred when C-17 crews were climbing using the vertical velocity hold function of the autopilot, and failed to transition to the climb on speed control function when their available thrust was insufficient to maintain an appropriate speed above stall. Other activations occurred when crews set a descent to a lower altitude using the autopilot but failed to engage the autothrottles, and upon reaching level flight at the bottom of the descent, the crews were unable to monitor the aircraft’s speed. One of ALS’s limitations is that notifications of activations only appear in the heads-up display, and if the crosscheck is slow or lazy, the notifications can be missed. The analysis also revealed that, in addition to the ALS activations, there were six stall warnings above 10,000 feet lasting longer than two seconds. To stay on the safe side, the pilot will need to execute continuous automation system monitoring and not fly at speeds below those calculated by the mission computer.

Based on long-term MFOQA analysis, some C-130J crews may have developed “reduced airspeed” flying techniques (likely to counter the threat of over-speeding the flaps at higher gross weights) that appear to rely on the dynamic Stall Speed Caret, along with the idea that C-130J engines produce near-instant thrust and lift because the engines do not require a lengthy “spool-up” period, and the four large propellers provide additional lift due to the “blown wing” effect. Add in the safety pads provided by the stick pusher and stall warning special alert, and as observed in the analysis, some C-130J crews appear to feel comfortable flying relatively close to the Stall Speed Caret. Unfortunately, this mindset fails to take into account the Dash-1 warning: “Stall warning speed increases or decreases dramatically with elevator inputs, power changes, flaps selection, or change in Gs.” Therefore, any unanticipated maneuvering can drive the aircraft instantly into a stall, whether it is caused by a tactical threat or the avoidance of a possible midair collision. Furthermore, a Dash-1 note highlights “the indicated airspeed at which the stick pusher activates should not be higher than the charted calibrated stall speed but may be as much as 12 knots lower due to variations in entry rate, power setting, center of gravity, stick pusher system installation, and airspeed system accuracy near stall speed”—meaning the stick pusher may not activate until after the aircraft has entered a stall. With these additional factors in mind, is this “close to the caret” flying technique creating a dangerous situation? Do aircrews realize that when they are operating that close to the stall, one or more unexpected forces acting on the aircraft could push them over the edge into disaster?

Finally, the C-130J analysts looked for all flights where the aircraft was not on takeoff or approach and flew slower than 120 knots-calibrated airspeed (KCAS). The analysis showed 96 events in the 12-month cycle ending October 2019, with one incident as slow as 104 KCAS. To counter this threat, most C-130J Stan/Eval pilots recommend 150 KCAS as the lower limit for a clean configuration C-130J, and 125 KCAS when partial flaps are selected for CDS airdrops. Although MFOQA analysis is not currently available for the C-130H until the completion of the Avionics Modernization Program, these speeds are also relevant to the H-model indicated airspeeds (please reference the applicable Flight Manual for lowest airspeeds at all specific configurations).

Similar speed-related threats are present in the KC-135 community that may have originated because of how KC-135 missions were executed during the Gulf War. During the Cold War, tanker crews trained to takeoff and meet their bomber receiver enroute to their targets, fuel was offloaded, and the mission was complete. In today’s environment (Gulf War forward), the KC-135 mission has morphed into a “hurry up and wait” mission. The crew speeds to a location to meet a fuel-starved receiver and then has to wait for the next call for fuel. Naturally, crews slow to save fuel so they can stay on station longer to support the war effort.

KC-135 crews also have the same challenges as other Mobility Air Forces crews in dealing with aircraft automation and the monitoring of said automation during the long wait between aerial-refueling tasks when boredom can lead to complacency. How bad is it? The KC-135 analysts looked at incidents that were below endurance speed, finding 44 events wherein the crews slowed more than 30 knots below endurance speed above 20,000 feet. Furthermore, 14 of these events were greater than 40 knots below endurance speed, with the most serious being 71 knots below. A good rule of thumb is not to slow below endurance speed. Going slower will, in actuality, not save any fuel. In addition, develop good Crew Resource Management and Threat/Error Management techniques to mitigate complacency caused by the lulls between mission activities.

As previously mentioned, none of these MDS event examples were associated with an approach. Individual sortie analyses of these non-approach events showed that some were attributed to either misset aircraft automation (where crews failed to ensure the automation was functioning as intended) or overly aggressive maneuvering during low-level operations. Analysis of the remaining sorties, however, showed that crews were adjusting power and pitch settings to maintain a specific speed. Did these crews understand just how close they were to stalling the aircraft?

The complexities of flying in the diverse and challenging arenas associated with aerial refueling and low-level operations subject crews to an intensely stressful environment where it is easy to lose situational awareness. Any additional “trick” in the bag that can be used to help reduce this stress will improve the odds of a desirable outcome. One of those tricks is knowing how slow the MDS should be operated, thus removing one of the possible “gotchas” and allowing the pilot to focus on more daunting obstacles to a successful and safe sortie.