I am studying the landing performances of the old Douglas DC-4 and there are a lot of different landing performance charts available. The pilots can use different flap settings during landing like 45, 35 and 25 degrees. For these different flap settings there are different charts. But there are also different charts for the Intended and Alternate destinations. Can someone explain to me why there are different charts for these?

How can the landing performance be different just based on intended vs. alternate? For example, in the charts for intended destination it says 60% of runway length and for the alternate destination 70% of runway length.


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Runway length = landing distance / 60%

For the DC-4 there are two sets of landing performance charts. The first one is the landing distance. The other one is the minimum effective runway length, for the intended and alternate airports separately.

From a 1982 NTSB safety recommendation:

The Safety Board recognizes that changes in the landing distance demonstration procedures during certification could result in penalizing the operational specifications of the airplane as they are presently determined using the existing minimum landing distance procedures. For actual line operations on dry runways, a safety margin is currently provided by the operational limitation of 14 CFR 121.195 which requires that the minimum effective runway length be the airplane's landing distance as determined during certification divided by 0.6 (or multiplied by 1.667). The Safety Board's accident investigation experience has not indicated to date that the actual runway lengths used in line operations for dry runways do not afford a proper level of safety. Safety Board recognizes that a change in the aircraft certification criteria specified in 14 CFR 25.101 and 25.125 will necessitate a corresponding review of the operational limitations in 14 CFR 121.195 so that operational specifications are not unjustifiably penalized. Of course, we are not suggesting that current runway length requirements be compromised to the detriment of present levels of safety.

The rules have changed since the DC-4 days. The related regulation I found applies to turbine aircraft, see: 14 CFR 121.195.

The 60% and 70% are an added factor of safety, which as the above text shows, limits the operational capability of the aircraft.

Dividing by 0.7 reduces the effective length required for the alternate compared to the intended stop, my understanding for the shorter runway length is to allow an easier selection of alternate airports, especially when the fuel maybe running low and the nearest airports are not of particularly long lengths.

The effective length charts make it clear that the 0.6 or 0.7 are already accounted for and need not be applied again. They would be manually applied if the crew are using the landing distance charts.

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The crew would start by the expected landing gross weight. It will be heavier at the intended stop due to carrying the alternate fuel. Then the pressure altitude, and finally via the reported headwind, the required effective length is known.

Related: How to calculate the DC-4 landing distance with this graph?

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