Y'know, it's always somethin'.
Seems like we just finished scraping ice and frost off the wings every morning, then we have to deal with density altitude, thunderstorms, microbursts and wind shear. This month we will discuss density altitude and next month we will tackle the other issues.
The semi-arid southwest region we fly in presents pilots with some unique challenges during the summer flying season. If this is your first summer flying in Colorado, you may be faced with some weather conditions you have never seen before. In addition, the fact that we are at approximately 6,000' MSL gives new meaning to the term high density altitude.
For many pilots accustomed to flying in parts of the country with field elevations closer to sea level, density altitude could become quite a challenge. Many factors contribute to a substantial reduction in aircraft performance at high altitudes; sometimes as much as 30%. On a 75°F day at Centennial, the density altitude is approximately 8,500 feet! This is the altitude the airplane thinks it's flying at, no matter what you think. Consequently, you should verify your calculations regarding aircraft performance (especially runway lengths) accordingly.
You will note that the Cessna 172 manuals do not provide a "normal" takeoff performance chart. The only performance charts provided are utilizing short field technique. Be careful when using these charts to determine takeoff distances. The ground roll numbers depicted could fool you into thinking that a normal takeoff at gross weight may be possible, when it is not. If you use normal takeoff technique, you must add a safety factor to the numbers depicted. This could mean your ground roll distance will be substantially longer, and in fact may make the takeoff inadvisable under some conditions.
At some mountain airports you will have a "one way ticket" if you land on a hot summer day. You can land, but you might not be able to takeoff. Lake County (Leadville) Airport comes to mind. The field elevation at Leadville is 9,927' MSL. On a 65°F day that means the density altitude is an astounding 12,350' MSL. That's close to the service ceiling of a 172. In addition, if you look at the performance charts in your 172 manual, you will note they stop at 8,000'. So that means if you land at Leadville that day, you have no way of knowing if you can takeoff safely. You have just become a test pilot. Are you sure you want to do that? If you are planning a flight to a high-altitude airport, you should consider flying early in the morning when it is cooler and completing your flight before noon. Otherwise, you may be taking a scenic tour of Leadville until it cools off later that evening or the next morning.
Next month we will take a look at thunderstorms, microbursts and wind shear. Our May 16th Safety Meeting will address microbursts. We have some great slides and video. Stop by and check it out.Q
The author would like to thank Jeff Ruetz and Larry Ramsdell (and everybody else and their brother) for their editorial comments.