Welcome Guest. Sign in or Signup

3 Answers

How do I calculate density altitude using these listed conditions?

Asked by: 3789 views , , , , ,
General Aviation, Student Pilot

This was on my Part 141 stage III exam and is the only one I cannot seem to understand. Apparently, the answer is 4,097 feet. Here is the question:

 

Determine the density altitude using the listed conditions.

Field elevation ........................ 1,400 feet

Pressure altitude ..................... 1,500 feet

Temperature ................................... 95oF

Altimeter setting .................. 29.82 in. Hg.

 

When I do this on my manual E6B computer, I get only 1,900 feet. I simply spin the scale until 35oC (converted from  95oF) is over 1,500 feet (the pressure altitude).

When I try to do mathematical using the formula, Density Altitude = Pressure Altitude + 120(Outside Air Temp. - ISA Temp), I get:

Density Altitude = 1400 + 120(35-2.8) = 5,264 feet.

I have no idea what I'm doing wrong. I fail to get the correct answer of 4,097 feet using both methods.

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

3 Answers



  1. Anderson on Nov 18, 2018

    Check your formula. ISA temp at 1500 is about 12 C. Also, you are using field elevation of 1400 instead of 1500 pressure alt.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes

  2. Best Answer


    ayavner on Nov 19, 2018

    so let’s check their work (and your E6B). First of all – to calculate Pressure Altitude, it is:

    (29.92-altimeter)*1000 = amount to add or subtract from Field elevation.

    so:

    (29.92″ – 29.82″)*1000 = 100′. The result is positive so you add it to the elevation. So yes, Pressure Altitude = 1400 + 100′ = 1500′. All good there. Now:

    Density Altitude = 120*(actual – ISA@elevation) + Pressure altitude

    ISA at 1400′ would be about (15 – 2*1.4) = 12.2 degrees at 1400′

    so

    (35 – 12.2)*120 + 1500 = DA

    2736 + 1500 = 4236′

    Not sure what the other choices are, but this value is what I get by hand-calculating it. there are other factors including humidity, dew point, etc which could account for a small difference such as this.

    Here is an online calculator which is even more precise, but I’ve never gotten the wrong answer on a test yet, even if sometimes it is just “the closest”. Knowing how to calculate it is handy in at least doing a reality-check.

    https://wahiduddin.net/calc/calc_da.htm

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes



  3. WarrenW on Nov 20, 2018

    Agree with ayavner. On the E6-B, I got just a hair over 4,000 or apparently 4,097 with 35C over 1,500. It’s all in the small window on the right side with the ‘AIRSPEED CORR.’ label underneath. Also check with the Part 141 Chief Flight Instructor – the 141 course with which I used to work had a manual with answers and explanations of all quiz questions.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.