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News Letter 13 September 2017

Good day All

Finally summer has arrived seems spring was forgotten in the process. Guys please be safe out there and remember to take Density Altitude into account with every flight. I’ve added a short write-up on the effects of Density Altitude released by the FAA recently.

Although density altitude is not a common subject for “hangar flying” discussions, pilots need to understand this topic. Density altitude has a significant (and inescapable) influence on aircraft and engine performance, so every pilot needs to thoroughly understand its effects. Hot, high, and humid weather conditions can cause a routine take-off or landing to become an accident in less time than it takes to tell about it.

Density Altitude Defined

Types of Altitude

Pilots sometimes confuse the term “density altitude” with other definitions of altitude. To review, here are some

• Indicated Altitude is the altitude shown on the altimeter.

• True Altitude is height above mean sea level (MSL).

• Absolute Altitude is height above ground level (AGL).

• Pressure Altitude is the indicated altitude when an altimeter is set to 29.92 in Hg (1013 hPa in other parts of the world). It is primarily used in aircraft performance calculations and in high-altitude flight.

• Density Altitude is formally defined as “pressure altitude corrected for nonstandard temperature variations.”

Why Does Density Altitude Matter?

High Density Altitude = Decreased Performance

The formal definition of density altitude is certainly correct, but the important thing to understand is that density altitude is an indicator of aircraft performance. The term comes from the fact that the density of the air decreases with altitude. A “high” density altitude means that air density is reduced, which has an adverse impact on aircraft performance. The published performance criteria in the Pilot’s Operating Handbook (POH) are generally based on standard atmospheric conditions at sea level (that is, 59° F or 15° C. and 29.92 inches of mercury). Your aircraft will not perform according to “book numbers” unless the conditions are the same as those used to develop the published performance criteria. For example, if an airport whose elevation is 500 MSL has a reported density altitude of 5,000 feet, aircraft operating to and from that airport will perform as if the airport elevation were 5,000 feet.

High, Hot, and Humid

High density altitude corresponds to reduced air density and thus to reduced aircraft performance. There are three important factors that contribute to high density altitude:

1. Altitude. The higher the altitude, the less dense the air. At airports in higher elevations high temperatures sometimes have such an effect on density altitude that safe operations are impossible. In such conditions, operations between midmorning and midafternoon can become extremely hazardous. Even at lower elevations, aircraft performance can become marginal and it may be necessary to reduce aircraft gross weight for safe operations.

Note: This document was adapted from the original Pamphlet P-8740-2 on density altitude.

Density Altitude

2. Temperature. The warmer the air, the less dense it is. When the temperature rises above the standard temperature for a particular place, the density of the air in that location is reduced, and the density altitude increases. Therefore, it is advisable, when performance is in question, to schedule operations during the cool hours of the day (early morning or late afternoon) when forecast temperatures are not expected to rise above normal. Early morning and late evening are sometimes better for both departure and arrival.

3. Humidity. Humidity is not generally considered a major factor in density altitude computations because the effect of humidity is related to engine power rather than aerodynamic efficiency. At high ambient temperatures, the atmosphere can retain high water vapour content. For example, at 35°C, the water vapour content of the air can be eight times as great as it is at 6° C. High density altitude and high humidity do not always go hand in hand. If high humidity does exist, however, it is wise to add 10 percent to your computed take-off distance and anticipate a reduced climb rate.

Check the Charts Carefully

Whether due to high altitude, high temperature, or both, reduced air density (reported in terms of density altitude) adversely affects aerodynamic performance and decreases the engine’s horsepower output. Take-off distance, power available (in normally aspirated engines), and climb rate are all adversely affected. Landing distance is affected as well; although the indicated airspeed (IAS) remains the same, the true airspeed (TAS) increases. From the pilot’s point of view, therefore, an increase in density altitude results in the following:

• Increased take-off distance.

• Reduced rate of climb.

• Increased TAS (but same IAS) on approach and landing.

• Increased landing roll distance.

Because high density altitude has particular implications for takeoff/climb performance and landing distance,

Pilots must be sure to determine the reported density altitude and check the appropriate aircraft performance charts carefully during preflight preparation. A pilot's first reference for aircraft performance information should be the operational data section of the aircraft owner's manual or the Pilot’s Operating Handbook developed by the aircraft manufacturer. In the example given in the previous text, the pilot may be operating from an airport at 500 MSL, but he or she must calculate performance as if the airport were located at 5,000 feet. A pilot who is complacent or careless in using the charts may find that density altitude effects create an unexpected—and unwelcome—element of suspense during take-off and climb or during landing.

If the airplane flight manual (AFM)/POH is not available, use the Koch Chart to calculate the approximate temperature and altitude adjustments for aircraft takeoff distance and rate of climb. At power settings of less than 75 percent, or at density altitude above 5,000 feet, it is also essential to lean normally aspirated engines for maximum power on takeoff (unless the aircraft is equipped with an automatic altitude mixture control). Otherwise, the excessively rich mixture is another detriment to overall performance. Note: Turbocharged engines need not be leaned for take-off in high density altitude conditions because they are capable of producing manifold pressure equal to or higher than sea level pressure.

World Aerobatic Championships

The World Aerobatic Championship is currently underway on home soil, Team SA is currently lying in fourth position overall but that could change quickly as the competition continues. Having the competition in South Africa has really helped the local guys to make it to the competition due to the massive costs of transporting their aircraft. Give It Hell guys and keep the flag flying high.

World Rally Flying World Championships

The World Rally Flying Championship is also on the go at the moment in Spain, we wish members of team South Africa all the best, Good luck Guys.

Airbus Helicopters launches Fleetkeeper Electronic Flight Folio for South Africa

Airbus Helicopters Southern Africa is offering its Fleetkeeper electronic technical logbook and electronic flight folio to customers and operators in South Africa, following its authorisation by the SACAA.

Fleetkeeper enables helicopter owners, pilots, maintenance and airworthiness managers to track flight hours, flight cycles and any other information regarding maintenance. It provides a helicopter’s airworthiness status to the pilots and records technical events during aircraft operation and any additional maintenance or repairs performed in-between scheduled base maintenance visits.

“We’re delighted to make this efficient application available to our South African customers and operators and intend to offer it in other parts of Southern and East Africa soon. We are currently in the process of securing similar authorisations from civil aviation authorities in other African countries, including Namibia and Kenya,” said AHZA Managing Director, Arnaud Montalvo.


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