Many pilots consider takeoff to be almost a no-brainer. Point the airplane down the runway and advance the power. A little footwork keeps the prop pointed in the right direction, then lift the nose.
Yet from an operational viewpoint, takeoff has proven to be one of the most critical phases of flight. NTSB accident statistics from 1998 – the latest available at this writing – show that general aviation airplanes of less than 12,500 pounds operated non-commercially suffered 262 takeoff accidents, 35 of which were fatal. That year, like each year in the five previous years, general aviation pilots averaged about 22 total and 3 fatal takeoff accidents per month, with no improvement as time passed. Clearly takeoff accidents are a problem in need of a solution.
Operationally, the takeoff phase is critical because this is when the airplanes weight is at its heaviest, airspeed and flight controllability at a minimum, and altitude at a premium. Combine these conditions with an unexpected event and you place both aircraft and occupants in jeopardy. In fact, a single unexpected event during takeoff could very well be the last link in the formation of an accident chain. The result can easily be bent metal, a post-crash fire and all that entails.
A dichotomy exists between the average pilots perceived need to prepare for takeoff and the real-world accident potential caused by the omission of crucial details. Takeoff accident causes reflect both omission of detail and lack of proper use and interpretation of performance charts. Surprisingly, mechanical failure is seldom the sole cause of an aircraft accident. In some cases, mechanical failure occurring in an otherwise manageable situation was compounded by the pilots improper response. The result is an accident that didnt have to happen.
Our review of takeoff accidents made it clear that a lack of applied aeronautical knowledge contributed quite frequently to the chain of events. Interestingly, those pilots who possess theoretical knowledge but cannot apply it to real-life situations are really no better off than pilots who lack that knowledge entirely.
The training maneuver mindset type of accident is an example of what happens when pilots do not apply their aeronautical knowledge. When a pilot is convinced that a maneuver or procedure is required merely to meet training or testing requirements, he or she has a tendency to subordinate its importance. The maneuver then assumes an inferior status in the pilots recall process. Thus, in an actual emergency it takes the pilot longer to recall the proper procedure and increases the likelihood the pilot will come up with the wrong answer or not get the right one quickly enough.
By shaking free of the training maneuver mindset, pilots sensitize themselves to the importance of each emergency maneuver – which makes recalling it that much easier. Of course, maintaining proficiency in the maneuver through practice and review is also important.
Developing the habit of properly determining airspeeds and distances enables the pilot to monitor takeoff performance more precisely. This heightened pilot sensitivity will result in earlier detection of anomalies and better reactions to the problem at hand.
Insidious Dangers
One of the most insidious dangers associated with the takeoff comes from not using checklists, or using them improperly. Regardless of how correct your performance figures are, omitting a step in configuring your airplane for takeoff renders those figures moot. For example, one pilot completed an operational check of the wing flaps, then tried to take off with the flaps in the landing position because he missed the checklist item that said Flaps to Takeoff Position.
Before taking the runway, the pilot must assure the airplane is in safe condition for flight. The omission of any one or combination of checklist items could result in an accident. Fortunately, most missed items are detected before they cause serious problems and the accident chain is broken.
Some airplanes are tolerant of taking off with improper configurations, and the pilot is left unsettled but still climbing away from the runway. But some mistakes eat up whatever performance margin exists.
How often proper use of a checklist has detected a missed item and broken the accident chain can never be known, but we do have some idea of the number of times the accident chain was not broken. Suffice it to say that during every pilots career they have missed an item or two because of distractions or poor memorization, but caught the omission by reading the checklist and avoiding an accident.
If this were not true, the takeoff and total accident rates would be much higher than they are. Therefore, the correct use of the Before Takeoff checklist cannot be emphasized strongly enough.
Additionally, the Before Takeoff checklist should be reviewed with the airplane stationary and without outside distractions or airplane movement. The greater the number of distractions, the greater the chance of error or omission.
Proper preflight planning takes into account weather conditions, weight and balance calculations and preflight inspection. But it also requires a check of the airplanes performance charts and the characteristics of the runway youll be using. For the purposes of this discussion, well assume the airplane involved is a piston-powered single of less than 6,000 pounds maximum certificated gross takeoff weight with a power-off stalling speed of 61 knots or less in the landing configuration (Vso). This qualification is necessary because airplanes certificated above that weight and stall speed have different certification standards. Most importantly, the majority of general aviation pilots fly airplanes of this weight and stall speed.
There are a number of variables that affect takeoff performance. To truly appreciate the information presented on takeoff performance charts, you must first understand how each variable affects the airplanes performance.
The performance charts contained in the manuals and handbooks of some older airplanes may lack one or more of these variables, however. This means pilots must apply their own aeronautical knowledge to fill in the blanks. But its possible to develop your own rules of thumb for filling in those blanks.
The two basic chart formats used are graphic and tabular. Regardless of format, the charts are segmented to accommodate atmospheric conditions such as temperature, pressure and wind; aircraft weight; and airport features such as runway length, surface condition and slope.
Atmospheric Conditions
A combination of pressure altitude and temperature produces a density altitude that affects wing lift, engine power output and propeller efficiency. With such a strong influence on aircraft performance it is imperative that pilots have an understanding of how density altitude is determined.
Density altitude is the result of mathematically converting the pressure altitude (what the altimeter reads when set to 29.92) and outside air temperature to a standard altitude. The standard atmosphere represents the mean or average properties of the atmosphere based on an average sea-level datum plane of a given pressure and temperature (density).
All aircraft performance is compared and evaluated in accordance with the density altitudes of this standard atmosphere. Therefore, the airplane will perform in accordance with the resultant density altitude regardless of its actual altitude.
An example would be an airplane at a sea-level airport where the pressure altitude is 500 feet and the temperature is 300 C (840 F). Checking these values against a density altitude conversion chart results in a computed density altitude of approximately 2,400 feet. The airplane will then perform as though it was at 2,400 feet msl.
Weight
At a heavier weight, and with the same thrust and power available, an airplane will require a greater distance to accelerate to takeoff speed. This is due to the reduction in rate of acceleration resulting from an increase of more pounds per horsepower and greater ground friction due to the heavier weight. Also, additional distance is needed to accelerate to the higher airspeed required to produce the greater lift necessary to support the heavier airplane.
Thus, the more pounds per horsepower, the greater the distance required to accelerate to lift-off airspeed and, consequently, the longer the takeoff distance. Conversely, at a lighter weight with fewer pounds per horsepower, the faster the acceleration and the shorter the takeoff distance.
The effect of weight on takeoff performance is represented on performance charts by various methods. When using a graphic performance chart, proceed from that point on the vertical gross weight baseline obtained when working the density altitude portion of the chart, and proceed diagonally down to the loaded weight value. From that point, proceed horizontally to the right to the wind baseline. Should the airplane be loaded to its maximum weight, you would proceed directly from the density altitude point on the weight baseline, then horizontally to the right to the wind baseline.
Wind
When using a graphic performance chart, go to the point where the weight value touches the wind baseline to determine wind effect. If there is a headwind component, follow the solid line diagonally down until it intercepts the headwind component value line. Then proceed horizontally to the right to the distance baseline, and read the ground roll or takeoff distance as appropriate for the chart being used.
If a tailwind component is present, proceed from the wind baseline diagonally up along the dashed line until you intercept the line emanating from the wind value located at the base of the graph. From that point you would proceed horizontally to the right to the distance baseline and read the ground roll or takeoff distance as appropriate for the chart used. It is evident that a tailwind increases the ground roll and total takeoff distance dramatically.
Notice that the dashed tailwind line has a steeper upward slope than the headwind downward slope for the same wind strength. There are two reasons for this: The headwind value is plotted at 50 percent of the actual headwind component, and the tailwind value plotted is 150 percent of the actual tailwind component. Therefore, the increased distance caused by a tailwind will be much greater than the decreased distance caused by a headwind of the same value. This requires that a steeper slope be used between wind values when plotting a tailwind component, resulting in a greater distance effect than with a comparable headwind component.
On a tabular chart, the wind effect on takeoff distance is made in accordance with the instructions contained in the Remarks or Notes section at the top of the chart. As an example in the Notes section of the Takeoff Distance chart for a 2,300-lb. maximum weight Cessna 172N, it states, 3. Decrease distances 10% for each 9 knots headwind. For operations with tailwinds up to 10 knots, increase distances by 10% for each 2 knots. This note confirms the same differences in headwind versus tailwind component adjustments on distances as previously explained on the graphical chart.
Both chart styles refer to a headwind or tailwind component. Since the wind seldom blows directly up or down the runway, the pilot must know and understand how a headwind or tailwind component is derived from a crosswind.
To determine headwind and crosswind component you need a chart that shows you or a calculator with sine and cosine functions. Take the cosine of the crosswind angle (cos 30 is 0.866), then multiply that by the velocity (40 knots) to get 34.6 knots of headwind component. To get the crosswind component, take the sine of the wind angle (sin 30 is 0.5) and multiply that by the velocity to get 20 knots of crosswind component.
Runway Length
Runway length requirements are determined by weight, atmospheric conditions (wind, temperature and density altitude), and surface conditions. Since you have already determined the effects of atmospheric conditions by using the takeoff charts, you can now consider the effect of runway length on takeoff strategy.
Operationally, either the runway length required is less than the runway length available, or its not. If the runway length required is within or less than the runway length available, no problem. However, should the runway length available be less than the runway length required, you will have to make adjustments to operate within the available runway length.
A short-field takeoff technique gives the pilot some control over the amount of runway required, but sometimes that isnt enough. Note that the takeoff charts in many operating handbooks stipulate that the short field technique is necessary to get those numbers. Many handbooks dont have any information on the distance required for standard takeoffs.
Pilots must realize that even with a given set of conditions over which they have no control – density altitude, wind, runway surface and length – they still have one option they can control: namely, weight. The airplanes weight can be adjusted in many ways to reduce the takeoff distance required. This reduction in weight may be in the form of fewer passengers, less baggage or cargo, a lighter fuel load or a combination of these.
The pilots choices for weight reduction always require an operational decision. If the flight is fuel critical, and no fuel stop can be made en route, passengers and/or baggage or cargo must be off-loaded. If a fuel stop can be made en route, then fuel can be off-loaded. Should the amount of surplus fuel removed not reduce the weight adequately, then a reduction in cargo or baggage would also be required. Generally the removal of passengers is a last resort, but in some cases it may be the first thing to consider, depending on the specific goals of the flight.
Lastly, if high temperatures are the primary cause of the runway length being insufficient – and you dont want to reduce fuel, baggage/cargo or passengers – the only choice is to delay takeoff until it cools down.
Pilots get caught in an off-loading dilemma because of the unexpected. Good flight planning generally prevents such situations, but poor flight planning encourages it. An example of poor flight planning would be an airplane arriving at a destination airport above its maximum landing weight. This situation usually results from the habit of topping off the tanks, or filling them to some predetermined level after each flight, without regard for fuel requirements for the next flight. Then, though knowingly determined necessary, not de-fueling before beginning the next flight.
The Runway Environment
Surface conditions affect takeoff distances because of their various coefficients of friction. Hard surfaces vs soft surfaces, wet runways vs dry runways, all produce distinct resistance values to the rolling tire. Hard, dry surfaces are the ideal from a braking, directional control and acceleration viewpoint. When the tires, especially the nose-wheel tire, have enough friction to act as good restraining points, they aid the pilot with directional control by tracking true and resisting displacement by crosswinds.
The term wet surface implies many things. It usually references any form of precipitation such as rain, snow, drizzle, or that is the result of precipitation, such as wet grass, mud, slush, ice, etc. But, regardless of its form, wetness allows a ridge to form in front of the tire that hinders acceleration and reduces tire-to-surface contact by allowing a film to develop beneath the tire and raising it off the runway surface. This reduces the coefficient of friction and directional control.
Although takeoff often seems like the easiest part of flying, proper takeoff planning can help keep you out of the bushes at the departure end. The pitfalls associated with takeoff performance are many, concealed by the fact that pilots usually take off from long runways under conditions where maximum performance is not required.
By understanding the relationship of the variables associated with takeoff performance you can develop a clear view of the total takeoff picture. But also remember that thorough preflight planning must be supported by proper checklist usage, and will do much to assure the safe outcome of any flight.
Also With This Article
Click here to view “Six of One, Half Dozen of the Other.”
Click here to view “Reading the Fine Print.”
-by Tom Oneto
Tom Oneto is a 13,000-hour ATP/CFII, Part 121 and corporate pilot and former Designated Pilot Examiner.
