Cruise performance 101

Noting the low airspeed, the pilots incorrectly attributed the speed decrease to ice accretion and activated the wing/stabilizer deicing boots. As activation of the deicing system causes the Phenom’s stall warning and protection system to reset to more aggressive warning thresholds in an expectation of ice contamination between boot cycles, an immediate stick pusher actuation occurred. The pilots’ attempt to recover from the stick pusher, made harder by the low thrust available in the thin, warm air, led to a loss of control and upset. Ultimately, the pilots fought to regain control through 6,000 feet of altitude loss before stabilized flight was achieved. Although the improper activation of the deicing system was the final link in the chain, the root cause of the incident was attempting flight higher than the aircraft could safely maintain with the given conditions.

For all the time spent on takeoff and landing performance planning in initial training programs, often very little or no time is spent on cruise planning. This is unfortunate, as historically, a failure to master the intricacies of cruise performance has often led to unsafe outcomes or outright accidents. Cruise altitude selection, in particular, is not always a straightforward consideration, especially on longer flights where the pilot is attempting to maximize fuel efficiency.

Setting the trap for high-altitude stalls is the fact that once out of training, jet pilots quickly learn that when it comes to cruise planning, higher is almost always better. Ignoring a small increase in engine efficiency at high altitude, jets cruise at an indicated airspeed that is a function solely of fuel flow. That is, for given factors the pilot cannot control, primarily weight, the airplane will fly at the same indicated speed for a given fuel burn, whether down low or up high. What does change with altitude, however, is the true airspeed that results from that “fixed” indicated speed.

So, while a Phenom 100, like the one in the stall event, cruising at a mid-weight and burning 600 total pounds of fuel per hour may indicate roughly 190 knots regardless of altitude, at 22,000 feet the corresponding true airspeed (TAS) will be 260 knots, while at 39,000 feet the TAS will be 360. What this means is the aircraft at 39,000 feet is flying 38 percent farther for every gallon of fuel burned. Seem like something for nothing? Most of the time it is. Excepting flight in strong winds that vary greatly with altitude, it is always more fuel efficient to fly higher than lower.

Given this fact, it is understandable that pilots flying a trip of any substantial distance often flight plan based on climbing to the ceiling of the airplane. Most of the time this works well, and the pilot can be lulled into a sense that a direct climb to the aircraft’s ceiling will always be practical. A trap lurks, however, in the form of nonstandard temperatures aloft.

The International Standard Atmosphere (ISA) is a model of temperature and pressure across altitude. It was created to provide a baseline reference and is used as the basis for performance calculations. Every pilot is familiar with the concept that as temperature rises, aircraft performance decreases. Jets are not immune to this basic fact, but as it relates to cruise performance, two factors in particular can cause confusion for jet pilots.

The first is that even a warmer than standard temperature at high altitude still seems pretty cold. It isn’t intuitive to a new jet pilot that a temperature of minus 40 degrees Celsius is actually extremely warm if encountered at 38,000 feet.

Second, when speaking of high altitudes, the outside temperature can be decoupled from geography and season in a way that seems illogical. For example, on a winter day in New England, a pilot may climb to 40,000 feet to find temperatures are 10 degrees C warmer than standard (ISA+10), while at that same moment the temperature over Florida at 40,000 feet is ISA minus 5. I’ve flown over Greenland many times, looking down at ice as far the eye can see, with temperatures aloft as high as ISA+21.

Ten to 20 degrees warmer than standard may not sound like much, but in the already thin air of 40,000 to 45,000 feet, the impact on cruise performance is profound. Consider that for a Cessna Citation CJ3/3+ at maximum weight, the difference between the runway length needed for a sea-level takeoff at 15 degrees C (ISA) versus 35 degrees C (ISA+20) is only 510 feet. But cruising at 45,000 feet, the same 20-degree swing is the difference between being able to sustain level flight and not.

For these reasons, a careful jet pilot needs to always intentionally review the forecast deviation from ISA when attempting cruise near the ceiling of their aircraft. This information should be applied when selecting, or accepting from ATC, a cruise altitude. For example, the smaller light jets with a relatively low thrust to weight ratio, such as the original Phenom 100 (pre-EV/EX) or the Citation Mustang, are capable of taking off at maximum takeoff weight (MTOW) and climbing directly to 41,000 feet, as long as the temperature aloft does not exceed ISA. But if the temperature at 41,000 feet is forecast to be minus-47 degrees C, or ISA+10, those same airplanes would need to burn off well over 1,000 pounds of fuel before being capable of sustaining cruise flight that high.

A pilot who ignores these limits risks a high-altitude stall as the Phenom 100 in South Africa experienced. The same airplane that may be climbing over 2,000 fpm at 5,000 feet msl will barely be making 200 fpm if it climbs through 40,000 feet at ISA+10. The temptation may be great for the pilot to raise the nose and climb at a lower speed to get up that last thousand feet. As the airplane slows, it crosses onto the back side of the power curve, where it will now climb at an even lower rate with further speed decrease. If the pilot fails to recognize the situation, the end result is inevitably an aerodynamic stall. Given that the aircraft is already putting out maximum power, recovery from such a stall will require a dramatic pitch-down input from the pilot, an action the pilot may not anticipate or perform adequately.

Fortunately, the above situation is preventable with basic preflight preparation. The manufacturer’s cruise performance charts show clearly what the weight, altitude, and temperature limits are for cruise. If a given set of parameters isn’t published in the cruise tables, it simply isn’t possible. Because these cruise performance charts are not typically published in the aircraft flight manual (AFM) where takeoff and landing performance information can be found, some pilots are unaware they even exist. Depending on the manufacturer and vintage of the airplane, cruise charts are found in supplemental publications called variously the pilot’s operating handbook (POH), operating manual (OM), or flight planning and performance manual (FPP).

Looking at the cruise performance tables for the Phenom 100, we see that after a maximum takeoff weight (MTOW) departure, cruise flight at 41,000 feet is only possible if temperature is ISA (or colder). At ISA+10 a Phenom taking off at MTOW and reaching top of climb at 10,000 pounds would be able to safely climb no higher than 38,000 feet. To reach 41,000 feet the aircraft would need to burn another 1,200 pounds of fuel, representing more than two hours spent in cruise.

Another consideration is the fact that while the aircraft, after enough time, may reach a weight where it is able to climb up a few more thousand feet, it may not be the most efficient choice to do so. We see that once weight has been reduced to 8,800 pounds, while the airplane may be capable of flight at 41,000 feet, after the last 1,000 feet of climb the low thrust output of the airplane will result in a 30-knot lower cruise speed, combined with a slight drop in per gallon efficiency. Slower speed for more fuel? The prudent pilot would recognize that, in this case, higher is not at all better. Neil Singer is a corporate pilot, designated examiner, and instructor in Embraer Phenoms and Cessna Citations. He has more than 10,000 hours of flight time with more than 20 years of experience as an active instructor.