Question about Commercial Test Prep

[SFCC/UND]

I not sure if you CFI keep your commercial test prep around, but I've been confused on number 5456. It ask (Refer to Figure 9.) Using a normal climb, how much fuel would be used from engine start to 12,000 feet pressure altitude.

Aircraft weight 3,800 lb
Airport pressure altitude 4,000 feet
Temperature 26 Celsius.

Possible answers
A-46 lbs
B-51 lbs
C-58 lbs

I read the graph and get an answer like 39 lbs then you have to apply note #2 Increase fuel by 10 percent for each 10 degree Celsius above standard temperature. This is where I get confused the answer helper box says Apply note # 2. (A temperature of 26 Celsius is +19 Celsius relative to the standard atmosphere at 4,000 feet.)

I don't understand how they got +19 Celsius from 4,000 feet when it's 26 Celsius. Any help please? Answer was C by the way.

[bLizZuE]

The temperature they list is probably surface, and they want you to use the standard lapse rates for altitudes...Just a guess.

[tgrayson]

Originally Posted by SFCC/UND

I don't understand how they got +19 Celsius from 4,000 feet when it's 26 Celsius. Any help please? Answer was C by the way.

Standard temp at sea level is 15C. Applying the standard lapse rate would mean that standard temperature at a PA of 4,000 = 15C - (4 * 2C) = 7C.

So the standard temp at 4,000 PA is 7C. However, they said it was 26C. That's (26C - 7C) = 19C above standard temperature for that PA. Many performance charts are indexed by degrees above standard, rather than any absolute temperature.