8. DISCUSSION—HAZARDS ASSOCIATED WITH RUNWAY OVERRUNS.(AC 91-79A)

8. DISCUSSION—HAZARDS ASSOCIATED WITH RUNWAY OVERRUNS.

In order to develop risk mitigation strategies and tools, it is important to identify hazards associated with runway overruns. A study of FAA and NTSB data indicates that the following hazards increase the risk of a runway overrun:

• Unstabilized approach;

• High airport elevation or high Density Altitude (DA), resulting in increased groundspeed;

• Effect of excess airspeed over the runway threshold;

• Airplane landing weight;

• Landing beyond the touchdown point;

• Downhill runway slope;

• Excessive height over the runway threshold;

• Delayed use of deceleration devices;

• Landing with a tailwind; and

• A wet or contaminated runway.

 

a. What is an Unstabilized Approach?

Safe landings begin long before touchdown. Adhering to the SOPs and best practices for stabilized approaches will always be the first line of defense in preventing a runway overrun. See Appendix 4, Unstabilized Approach Case Study, for an example of an unstabilized approach that led to a runway overrun.

b. High Airport Elevation.

High airport elevation or high DA results in a higher true airspeed (TAS) and groundspeed, and a corresponding longer landing distance, compared to low airport elevation or low DA.

NOTE: For example, at 1,000-ft airport elevation, with a Landing Distance Factor (LDF) (see Appendix 1, Suggested Procedures and Training Information) of 1.05 to 1.10 (depending on runway condition), the crew should apply to the landing distance achieved at a sea level airport elevation. The AFM/POH usually includes the adjustment for this factor.

c. The Effect of Excess Airspeed.

The pilot must be aware of airspeed during the approach and of the referenced landing airspeed (VREF)/airspeed, plus wind gust adjustments, which is maintained until over the runway threshold. An excessive approach speed may result in an excessive speed over the runway’s threshold, which may result in landing beyond the intended touchdown point as well as a higher speed from which the pilot must bring the airplane to a stop. (Refer to FSF ALAR Briefing Note 8.3 and Boeing’s Takeoff/Landing on Wet, Contaminated, and Slippery Runways.)

          (1) FSF ALAR Briefing Note 8.3. A 10-percent increase in final approach speed results in a 20-percent increase in landing distance. This assumes a normal flare and touchdown (i.e., not allowing the airplane to float to bleed excess airspeed).

          (2) Example. Runway length available is 5,000 ft and the airplane’s AFM/POH certified landing distance may be up to 3,000 ft at the correct airspeed at the threshold crossing point. However, given a 10-percent increase in airspeed over the runway threshold and increasing the landing distance by 20 percent, the resultant operational landing distance is now 3,600 ft (1.20 × 3,000).

d. Airplane Landing Weight.

Any item that affects the landing speed or deceleration rate during the landing roll will affect the landing distance. The effect of gross weight on landing distance is one of the principal items determining the landing distance of an airplane. One effect of an increased gross weight is that the airplane will require a higher landing speed. When one considers minimum landing distances on a dry runway, braking friction is the main source of deceleration. The minimum landing distance will vary in direct proportion to the gross weight.

NOTE: For example, a 10-percent increase in gross weight at landing would result in a 5-percent increase in landing velocity and a 10-percent increase in landing distance. (Refer to the Pilot’s Handbook of Aeronautical Knowledge for more information.)

e. Landing Beyond the Intended Touchdown Point.

AFM/POH distances are based on a touchdown point determined through flight testing procedures outlined in AC 23-8, AC 25-7, and AC 25-32. If the airplane does not touch down within the air distance included in the AFM or POH landing distance, it will not be possible to achieve the calculated landing distance.

f. Downhill Runway Slope.

Refer to FSF ALAR Briefing Note 8.3. Runway slope (gradient) has a direct effect on landing distance. For example, a 1 percent downhill slope increases landing distance by 10 percent (factor of 1.1). However, this effect is accounted for in performance computations only if the downhill runway slope exceeds 2 percent. See Appendix 1, Table 1-3, Sample Increase in Landing Distance Computation If Runway Slope Is Not Included in the AFM/POH Landing Performance Data, With a Runway Length of 7,000 Feet, if runway slope is not included in the AFM/POH landing performance data.

g. Excessive Height Over the Runway Threshold—Threshold Crossing Height (TCH) Greater Than 50 Feet (Excess TCH). The certified landing distances furnished in the AFM are based on the landing gear being at a height of 50 ft over the runway threshold. For every 10 ft above the standard 50-ft threshold height, landing air distance will increase 200 ft.

NOTE: For example, TCH of 100 ft increases the landing distance by about 1,000 ft (50 additional ft divided by 10 = 5 × 200 ft landing distance increase per each 10 ft above 50 ft TCH = 1,000 ft additional landing distance). (Refer to FSF ALAR Briefing Note 8.3.)

h. Delayed Use of Deceleration/Maximum Braking.

(1) For those airplanes so equipped, deceleration devices consist of spoilers, thrust reversers, and brakes. The touchdown point is important since the wheel brakes are much more effective in retarding the airplane than the air drag during the airborne part of the landing distance. The sooner the airplane touches down and starts braking, the shorter the total distance will be. The FSF ALAR Task Force found that delayed braking action during the landing rollout was involved in some of the accidents and serious incidents in which slow/delayed crew action was a causal factor. The FSF Runway Safety Initiative (RSI) team found that improper use and malfunction of speed brakes, wheel brakes, and reverse thrust were significant factors in a number of runway excursion landing accidents.

(2) Prompt and proper operation of all means of deceleration has a major influence on landing distances. Spoilers greatly decrease lift, dump the weight on the wheels, and thereby make the brakes effective. It should be noted that manual spoilers, operated by the pilot, involve a delay. Even 2 seconds at speeds of 200 ft/second (118 knots (kts)) can increase the stopping distance by almost 400 ft. Landing distance data in the AFM is typically based on a time increment of 1 second between successive actions to manually deploy/engage the deceleration devices (see Figure 1, Assumed Landing Time Delays in Deriving the Scheduled Landing Distance). A conservative approach is to add 200 ft to the landing distance for every second in excess of 2 seconds to deploy the airplane’s deceleration devices. A prudent pilot will make a reasonable adjustment to the airplane’s landing distance for any delay in employing the airplane’s deceleration devices. Figure 2, Braking Devices on Stopping Energy and Stopping Distance, shows the relative effectiveness of each of the deceleration devices during the landing roll (refer to FSF ALAR Briefing Note 8.4).

(3) Be conservative and add 20 percent to the rollout distance if the pilot does not maintain maximum braking until the airplane reaches a full stop. Otherwise, if available, use AFM data for less than maximum braking.

(4) For airplanes that do not have antiskid brakes, spoilers, or thrust reverse, caution should be exercised. Excessive braking can lead to causing a tire failure or cause a skidding condition, leading to a runway excursion. Therefore, flying a stabilized approach and timely application of deceleration devices are the keys to a safe landing.

NOTE: For example, available runway of 5,000 ft, AFM landing distance of 3,000 ft, at correct speed, and at 50 ft TCH, a total of 3 seconds to deploy the airplane’s deceleration devices, results in 1 second over the AFM landing distance, assumed 2 seconds to deploy deceleration device will result in an additional 200 ft operational landing distance, for a total of 3,200 ft.

i. Landing With a Tailwind—Effect of a Tailwind on Landing Distance.

The effect of a tailwind on landing distance is significant and is a factor in determining the landing distance required. Given the airplane will land at a particular airspeed, independent of the wind, the principal effect of a tailwind on operational landing distance is the change in the groundspeed at which the airplane touches down.

(1) The effect of a tailwind will increase the landing distance by 21 percent for the first 10 kts of tailwind. (Refer to the Pilot’s Handbook of Aeronautical Knowledge and the aircraft’s AFM/POH data to determine if tailwind landing data is available for the airplane.)

(2) Tailwind landings affect all types of airplanes. For transport category airplanes, the effect of tailwind is shown in the AFM landing distance information. For small airplanes, tailwind landing data may not be provided. The FAA Small Aircraft Branch provided the following tailwind performance information for a few small airplanes:

• Cessna 150 and 152, note on the landing distance chart, “for operation with tailwinds up to 10 knots, increase distances by 10 percent for each 2 knots.”

• TMB 850, note under landing distance table to “increase total distances of 30 percent for every 10 knots of tailwind.”

• The Cirrus and Columbia are two very popular piston airplanes. The Cirrus uses the same note in the chart as Cessna. The Columbia is like the Diamond airplane and offers factors for grass, but not tailwinds.

NOTE: Remember to account for the effect of a tailwind on landing distance, whether you are flying a large or small airplane.

(3) Tailwind example: Available runway of 5,000 ft, AFM landing distance of 3,000 ft, TCH at 50 ft and at the correct airspeed with a 10 kts tailwind results in an increase in the operational landing distance of 21 percent. This increase equates to an additional 630 ft, which increases the operational landing distance to a total of 3,630 ft.

j. A Wet or Contaminated Runway.

Landing distances in the manufacturer-supplied AFM provide performance in a flight test environment that is not necessarily representative of normal flight operations. For those operators conducting operations in accordance with specific FAA performance regulations, the operating regulations require the AFM landing distances to be factored to ensure compliance with the predeparture landing distance regulations. These factors should account for pilot technique, wind and runway conditions, and other items stated above. Pilots and operators should also account for runway conditions at the time of arrival (TOA) to ensure the safety of the landing. Though the intended audience of SAFO 06012 is turbojet airplanes, it is highly recommended that pilots of non-turbojet airplanes also follow the recommendations in SAFO 06012.

(1) The SAFO urgently recommends that operators develop a procedure for flightcrews to assess landing performance based on conditions actually existing at the TOA, as distinct from conditions presumed at time of dispatch. Those conditions include weather, runway conditions, the airplane’s landing weight, landing configuration, approach speed, and whether the flightcrew deploys deceleration devices in a timely manner.

(2) Once the actual landing distance is determined, an additional safety margin of at least 15 percent should be added to that distance. Except under emergency conditions, flightcrews should not attempt to land on runways that do not meet the assessment criteria and safety margins as specified in SAFO 06012.

(3) A safety margin of 15 percent should be added, and the resulting distance should be within the runway length available. The FAA considers a 15 percent margin to be the minimum acceptable safety margin.

NOTE: The flightcrew should not apply this 15 percent safety margin to the landing distance determined for compliance with any other OpSpec/MSpec requirement.

(4) Know you can stop within the Landing Distance Available (LDA). The cumulative effect of the conditions that extend the airplane’s landing distance, plus the 15 percent safety margin, can be a substantial increase to the AFM/POH data, unless the pilot is aware of the items presented and possesses the knowledge and flying discipline to mitigate the risk of a runway overrun (see Table 1 below).

NOTE: This table is applicable to all landings. This table is a listing of elements that affect the landing distance of an aircraft, and it expresses risk management and critical thinking.