Know Your Numbers
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Had you going there for a second or two, didn’t I? Bet you thought I was talking about medical stuff! Indeed, important as your personal health numbers are, knowing your airplane numbers will keep you alive a lot longer as opposed to the former (at least in many cases).
So what’s the big deal about aircraft numbers, you ask? Well, just about everything. Let’s pull this thing apart and take a closer look at how those numbers impact you and your passengers.
Since I’m primarily speaking to pilots who operate aircraft in and out of high-elevation airports (4,000 feet and above), my real purpose is to discuss the effects of density altitude and how it impacts the performance of light general aviation aircraft. My hope is that this article will positively influence some to pay attention to those numbers and, in doing so, prevent a catastrophic event from occurring. Specifically, loss of life and/or damage to aircraft.
Spring is right around the corner, and with that comes warmer temperatures and higher risks of density altitude-related accidents.
So allow me, if you will, to “don my CFI hat for a spell” and discuss this very important topic. Let’s start with 14 CFR 91.103 – Required Preflight Actions.
Take notice that the noun “Actions” is plural, meaning more than one. Think of it as your “due diligence” or duty to perform! It’s non-negotiable and absolute!
In plain English, it means “you have no choice in the matter, you have to do it!”
To make it easy, consider the acronym NWKRAFT, which stands for:
- NOTAMs (Notice to Airmen)
- Weather Reports and Forecasts
- Known ATC Delays
- Runway Lengths of Intended Use
- Alternatives Available (if the flight cannot be completed)
- Fuel Requirements
- Takeoff and Landing Performance Data
To stay on topic and “on point,” our discussion will focus on three items from the list above. The first: weather. The second: runway lengths of intended use. And the third: takeoff and landing performance data.
Moving forward, what’s important with weather other than everything? Glad you asked.
While the wind direction and velocity will help us determine the proper runway to depart from (or if we should even depart at all), I’m really drilling down on temperature, dewpoint, altimeter (barometric pressure), and density altitude.
Those are the “Fab Four.” They’ll make you or break you “if you let them.”
Density altitude is pressure altitude corrected for non-standard temperature.
What’s standard? For temperature, it’s 59°F/15°C, and for pressure, it’s 29.92 inches of mercury (inHg) or 1013.2 Mb, all at sea level and considered a “standard day.”
Because we’re not at sea level, we have to correct for non-standard conditions. The bottom line is this: if the barometric pressure or altimeter falls below 29.92 inHg, your pressure altitude increases by approximately 100 feet for every 0.10 inch below 29.92.
Example: If the current altimeter setting is 29.12, you would add 800 feet (29.92 – 29.12 = 0.8 × 1,000) to the airport elevation to arrive at the pressure altitude. Setting 29.92 in the Kollsman window should net a similar result.
Adding 800 feet to the airport elevation of 6,878 feet (KFLY) gives you a pressure altitude of 7,678 feet.
Now for the fun part. Remember back to what density altitude was? DA = PA corrected for non-standard temperature (59°F/15°C).
Let’s say the current temperature is 82°F/28°C. The formula is:
DA = PA + {120 × (OAT – ISA Temp)}
It’s pretty straightforward other than figuring out ISA temperature.
ISA is called International Standard Atmosphere, which is the standard temperature of 59°F/15°C at sea level. Our airport elevation is way above sea level at 6,878 feet, so ISA temperature decreases by approximately 3.5°F (2°C) for every 1,000 feet above sea level.
If we take our field elevation of 6,878 feet and divide by 1,000, we get 6.878. We can round that up to 6.9 and multiply that by 2 (remember, the temperature decreases by 2°C for every 1,000 feet of elevation) which equals 13.8.
Now subtract 13.8° from 15°C (standard temperature) to arrive at your ISA temperature of 1.2°C.
Now we can solve for density altitude.
DA = PA [7,678] + {120 × (OAT [28°C] – ISA Temp [1.2°C])}
So, 28°C – 1.2 = 26.8°C × 120 = 3,216.
If you’re wondering where the 120 in that equation came from, it means that for every additional 1°C increase in temperature, the density altitude increases by 120 feet.
Remember, as the air temperature heats up, it becomes thinner and less dense. Engines make less power, wings produce less lift, and propellers are less efficient.
Now add that 3,216 feet to your PA of 7,678 feet and your density altitude is a whopping 10,894 feet!
Now try plugging that number into your aircraft’s performance chart, and after performing your weight and balance calculations, see how much runway you’ll require for takeoff. Chances are your POH doesn’t even calculate density altitude that high.
I essentially showed you how to calculate density altitude the “old-fashioned way”; you can get that same number easily from any number of aviation weather resources.
By taking the density altitude and incorporating it into runway lengths of intended use and takeoff and landing performance data, you now have all the information you need to make an informed “go/no-go” decision.
But always remember this: those charts were developed using factory-new aircraft that were being flown by test pilots. Our 40- to 60-year-old airplanes may not perform as well, especially in high elevations.
The solution? Add a “fudge factor” in. Some pilots choose to add a significant margin—sometimes 50%—to published performance numbers.
After reviewing the remaining NWKRAFT items, you’ve satisfied the requirements of 14 CFR 91.103.
In comparison to the PA/DA calculations, it may seem like I’ve “glossed over” the runway lengths and takeoff and landing performance data. Perhaps, but pressure altitude and density altitude are the real “drivers” in terms of extrapolating the correct performance numbers for your particular aircraft.
Without knowing PA or DA, you essentially know nothing. It’s nothing but a guess.
Do the homework, pull out the POH, and plug the numbers in where they belong. It’s that easy.
In closing, let me say this: Every single year, there are several fatal accidents resulting from high density altitude and decreased aircraft performance.
Remember—density altitude is a killer!
Your passengers are trusting you to make the right decision. Know your numbers, know your aircraft, and know your limitations!
It’s better to be on the ground wishing you were flying than being in the air wishing you were still on the ground.
Fly safe!
With over 2,000 hours of instruction—more than 800 as a CFII—Tom brings extensive experience to both student pilots and advanced instrument training. He is an active member of the Colorado Pilots Association, EAA Chapter 72 in Falcon, CO, and SAFE, and serves as a FAASTeam Representative for the Denver FSDO, conducting monthly in-person safety seminars throughout Southern Colorado.
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