Density Altitude Takeoff Note

[CoffeeCan]

My wife and I recently completed our first vacation XC of 2021 in our M20K,from KABI to KVGT and home again, to visit family. The return flight, at 17.5K MSL, was a delightful 4.2 hour cruise, but of course the outbound leg against headwinds between 8500 and 12,500 MSL was a tad longer (6.5 hr). We made a fuel stop at Moriarty NM (field elev. 6420) on the westbound flight. 

I must confess I made a significant error in failing to calculate takeoff distance after fueling up at Moriarty. I’m used to flying several hundred pounds lighter at 1000’, which rarely requires more than 1600’ for the takeoff roll. So with my brain firmly planted in the mindset that takeoffs are on rails, I failed to watch my airspeed after seeing the needle pass 60 KIAS. At home field elevation the airplane starts trying to leap off the runway within 3-4 seconds after passing 60. My reptilian brain told me to start pulling back on the yoke, but instead of the anticipated brisk liftoff, we stayed firmly planted on the runway. I pulled harder and the nose came up, but an annoying buzzing noise started which I failed to recognize as the stall warning. We lifted off but immediately began drifting left and the controls seemed heavy instead of the usual lightness I have come to expect. 
 

My human brain then realized things were not right. I cut power and pushed the yoke forward and we bumped back down onto the runway (on the mains, nose-up attitude was maintained) and while the airplane bled off airspeed my heart rate jumped to compensate. After a brief pause to make sure neither we nor the airplane had suffered any apparent damage, we taxied back to the threshold and my wife and I did a thorough review of possible things that might have gone wrong. It dawned on me that while our turbocharged engine thinks it’s at sea level, the prop and wings are working with thin 6500’ air density. If that were the case, I reasoned, I had tried to rotate too early. We decided to perform a high-speed taxi on the runway to be positively sure all the airplane’s systems were OK. 
 

We did a second run-up, triple-checked our takeoff settings, and did our fast taxi while watching the airspeed and all warning lights very carefully. Sure enough, above 67 KIAS the nose tried to lift and the controls felt light and responsive. Moreover, the annoying buzzing sound failed to present itself. However: it took a LOT longer to reach takeoff airspeed than I expected. Rather than take off, we taxied back and reviewed everything one more time. We then put in our takeoff flaps and attitude, applied full power (yep, 40” MP came up just like always) and rolled until the bird lifted off at 67 KIAS. We climbed out of the pattern by textbook numbers without incident and our turbocharged engine took us up to cruise altitude exactly as it always does. 
 

I share this embarrassing incident as a way of reminding myself (and maybe you) that it doesn’t matter how much I am “used to” flying my airplane, new and different conditions require careful preparation and operations. Sure, I did our W&B calculations prior to departure, watched my fuel and kept up with the fuel normalizer en route, etec, etc, etc, but one crucial oversight about takeoff rolls at higher altitudes could have been disastrous. 
 

My POH tells me that my takeoff roll at standard temperature at 6500 MSL is closer to 2200’ than the 1600’ I have become accustomed to down here in the flatlands. That extra 600 feet takes a lot longer than you’d think it does. I got careless in this case and made an error that, if I had persisted in being stupid, could have seriously wrecked our airplane or even killed us.  Stalls on takeoff killed over 80 people in GA in 2020. Density altitude was a contributing factor in a significant number of those fatal crashes. 
 

I hope telling on myself in this way serves to remind other Mooney pilots that our high performance aircraft are still subject to the same laws of aerodynamics as every other airplane.  Watch density altitude. It can bite. 

Edited May 12, 2021 by CoffeeCan

[gsxrpilot]

Wow! I'm glad everything worked out alright!

I'm sure there will be a lot of responses on this and lots of advice. I'm pretty sure my take on this will be panned by many as well. But in my cockpit I like to fly based on the feel of the airplane and less on the numbers. As you noticed, the wing will tell you when it's ready to fly. It will take longer to get there with high DA, and you'd better be sure you have enough runway. But there's no use trying to make it fly before it's ready. So notice the feel of the wing ready to fly, as it will always be the same regardless of DA.

Experience helps a lot with this as well. I've been fortunate to be able to get lots of experience with various high DA take offs. I've taken off with DA's nearly 12,000 ft. I've been able to note the take off roll to see how long it COULD be in a heavy and high DA situation. My 252 has never still been on the ground at 2000 ft. So that tells me that as long as I have say 4000 ft of runway, I can easily get out, regardless of the DA. Fortunately most of the high altitude airports in the West, have long runways. 

If I'm making full power, (I'll know in the first 100 ft or so), I know I won't be on the ground at 3000 ft. But I'll let it stay on the ground until the wing tells me it's ready to fly.

[donkaye]

This is a good reason to practice soft field takeoffs.  Mastering this takeoff will teach you how the airplane behaves in the most critical situation and how you SHOULD behave is such a situation.  The airplane will leave the ground before it really wants to fly because of ground effect.  So, it is important to STAY IN GROUND EFFECT until you reach Vx, then, and only then, begin your climb out.  And ground effect means GROUND EFFECT, as close to the ground as possible.  Remember induced drag is reduce by almost 50% when your distance above the ground is 10% of the wing length.  On our 36 foot wing that is 3 feet, and 3 feet is the height of the wing above the ground when stationary.  So STAY LOW!  I have found  that most people in training think they are going to hit the prop and after the takeoff quickly get out of ground effect.  Train yourself to stay low!

Also, high DA's also mean flying a wider pattern to account for the higher true airspeed.

[carusoam]

Nice work CC!

Great logic.

Thank you for sharing your experience...

Its that time of year again to remember the affects of DA.

Runways get shorter, trees get taller, and we remember Patrick kindly for reminding us....

Know before you go...

Great reminder especially for those who are new to aviation... or have a new 2U plane...

Best regards,

-a-

[kortopates]

Yes, it seemingly takes forever even in our Turbo charged aircraft to get to Vy to climb out from a high density altitude airport. This is why on the third day of my transition training course I take the candidate up to a high density altitude airport such as Bg Bear. Last week training at Havasu City we went up to Flagstaff. (of course we couldn't do that if we weren't in the western US). But in so doing, the new owner can experience the differences before going out on their own. More than once, especially on a less forgiving NA aircraft, I've had to push nose down right after rotate to keep the aircraft from quickly stalling. The soft-field high density altitude takeoff that Don describes above is the only way to get a normally aspirated aircraft out of ground effect when the density altitude gets above the aircraft gear down service ceiling! (A value not published in every aircraft including the Mooney) So for an NA aircraft, when obstacles allow, we'll keep the aircarft in ground effect after retracting the gear at Vx and start climbing after accelerating to Vy. Vy will give some airspeed buffer on the right side of the power curve for the turbulence we are likely to experpeince climbing out of a mountainous HighDA airport. The Turbo is more forgiving but patience in getting up to the required speed and taking advantage of ground effect are still very important.

[jaylw314]

It's still hard not to be jealous of you guys with turbo motors.  I recall taking off at about 5000' DA with four people.  The takeoff was easy with a 30 knot headwind, but the climb out felt pretty alarmingly slow!

[Ragsf15e]

2 hours ago, CoffeeCan said:

My wife and I recently completed our first vacation XC of 2021 in our M20K,from KABI to KVGT and home again, to visit family. The return flight, at 17.5K MSL, was a delightful 4.2 hour cruise, but of course the outbound leg against headwinds between 8500 and 12,500 MSL was a tad longer (6.5 hr). We made a fuel stop at Moriarty NM (field elev. 6420) on the westbound flight. 

I must confess I made a significant error in failing to calculate takeoff distance after fueling up at Moriarty. I’m used to flying several hundred pounds lighter at 1000’, which rarely requires more than 1600’ for the takeoff roll. So with my brain firmly planted in the mindset that takeoffs are on rails, I failed to watch my airspeed after seeing the needle pass 60 KIAS. At home field elevation the airplane starts trying to leap off the runway within 3-4 seconds after passing 60. My reptilian brain told me to start pulling back on the yoke, but instead of the anticipated brisk liftoff, we stayed firmly planted on the runway. I pulled harder and the nose came up, but an annoying buzzing noise started which I failed to recognize as the stall warning. We lifted off but immediately began drifting left and the controls seemed heavy instead of the usual lightness I have come to expect. 
 

My human brain then realized things were not right. I cut power and pushed the yoke forward and we bumped back down onto the runway and while the airplane bled off airspeed my heart rate jumped to compensate. After a brief pause to make sure neither we nor the airplane had suffered any apparent damage, we taxied back to the threshold and my wife and I did a thorough review of possible things that might have gone wrong. It dawned on me that while our turbocharged engine thinks it’s at sea level, the prop and wings are working with thin 6500’ air density. If that were the case, I reasoned, I had tried to rotate too early. We decided to perform a high-speed taxi on the runway to be positively sure all the airplane’s systems were OK. 
 

We did a second run-up, triple-checked our takeoff settings, and did our fast taxi while watching the airspeed and all warning lights very carefully. Sure enough, above 67 KIAS the nose tried to lift and the controls felt light and responsive. Moreover, the annoying buzzing sound failed to present itself. However: it took a LOT longer to reach takeoff airspeed than I expected. Rather than take off, we taxied back and reviewed everything one more time. We then put in our takeoff flaps and attitude, applied full power (yep, 40” MP came up just like always) and rolled until the bird lifted off at 67 KIAS. We climbed out of the pattern by textbook numbers without incident and our turbocharged engine took us up to cruise altitude exactly as it always does. 
 

I share this embarrassing incident as a way of reminding myself (and maybe you) that it doesn’t matter how much I am “used to” flying my airplane, new and different conditions require careful preparation and operations. Sure, I did our W&B calculations prior to departure, watched my fuel and kept up with the fuel normalizer en route, etec, etc, etc, but one crucial oversight about takeoff rolls at higher altitudes could have been disastrous. 
 

My POH tells me that my takeoff roll at standard temperature at 6500 MSL is closer to 2200’ than the 1600’ I have become accustomed to down here in the flatlands. That extra 400 feet takes a lot longer than you’d think it does. I got careless in this case and made an error that, if I had persisted in being stupid, could have seriously wrecked our airplane or even killed us.  Stalls on takeoff killed over 80 people in GA in 2020. Density altitude was a contributing factor in a significant number of those fatal crashes. 
 

I hope telling on myself in this way serves to remind other Mooney pilots that our high performance aircraft are still subject to the same laws of aerodynamics as every other airplane.  Watch density altitude. It can bite. 

Standard temp at 6500’ is 35 degrees f.  I suspect where you were was warmer.  Density altitude was likely much higher.  I have an NA airplane so I generally don’t takeoff after 9am from high airports in the summer.  Flying out west it’s easier to remember this, but you learned a good lesson you likely won’t forget.  Just know the temp as well because even a measly 3000’ altitude airport at 95 degrees will get your attention.

[EricJ]

Kudos for handling that carefully.   Caution is a good attribute in this business.

I kinda did the opposite yesterday.    I went to Taylor, AZ, to buy some cheap gas but their pump didn't like my card for some reason.   The wx bot said the DA was 7700', so I expected a bit of a lazy takeoff.    I pulled the mixture back a bit to my normal t/o EGTs on the takeoff roll and the thing just about leaped off the ground.   Usually leaning doesn't add that much power so I was wondering what was going on, and then the initial climb was pretty slow, as I had expected.

Then it dawned on me that I had way less fuel on board than I usually do.   Once it accelerated it climbed really well. 

[CoffeeCan]

8 minutes ago, Ragsf15e said:

Standard temp at 6500’ is 35 degrees f.  I suspect where you were was warmer.  Density altitude was likely much higher.  I have an NA airplane so I generally don’t takeoff after 9am from high airports in the summer.  Flying out west it’s easier to remember this, but you learned a good lesson you likely won’t forget.  Just know the temp as well because even a measly 3000’ altitude airport at 95 degrees will get your attention.

Thanks for the correction, you're spot on. I had based my calculations on the day on standard temp at sea level. 

FWIW, I ran the numbers again on a DA calculator, and in the conditions that day the DA was 8167'.  

[CoffeeCan]

2 hours ago, gsxrpilot said:

Wow! I'm glad everything worked out alright!

Experience helps a lot with this as well. I've been fortunate to be able to get lots of experience with various high DA take offs. I've taken off with DA's nearly 12,000 ft. I've been able to note the take off roll to see how long it COULD be in a heavy and high DA situation. My 252 has never still been on the ground at 2000 ft. So that tells me that as long as I have say 4000 ft of runway, I can easily get out, regardless of the DA. Fortunately most of the high altitude airports in the West, have long runways. 

 

Thanks, Paul. My experience at high altitude airports is very limited, which is part of why I'm planning on taking a 2-day mountain flying course in NM this summer. I have close to 800 hours TT at this point, but that is almost all in the flat lands. FWIW, Rwy 18 at Moriarty is 6201' long, which bears out your comment. 

[N201MKTurbo]

When I first started flying Mooneys I lived in Denver. Denver was about the lowest elevation airport I operated at back then. I changed my way of planning my takeoffs from a high elevation airport. Instead of figuring out how much runway it takes to take off, you only need to know one number, the distance to abort. It doesn’t change much with elevation. I just used 1000 feet. All you have to do is look at the airport diagram and pick a landmark 1000 feet from the end. If your wheels are not off the ground by the landmark, throttle to idle and hit the brakes. 
 

Most high elevation airports have very long runways. Try Leadville on a warm summer day,  you will think you are going to drive over the mountains.

[CoffeeCan]

1 hour ago, carusoam said:

Nice work CC!

Great logic.

Thank you for sharing your experience...

Its that time of year again to remember the affects of DA.

 

Thanks, sir! 

Initially, I had no intention of exposing my ineptitude on here. But yesterday I watched a YouTube video by Dan Gryder in which he quoted the GA fatal accident stats last year... the number of deaths associated with low-speed stalls in the pattern (both takeoffs and landings) was shocking. Given the fact that I know my aircraft's Minimum Maneuvering Speed, have it placarded on my panel, and watch it like a hawk when I'm in the pattern, this airspeed SNAFU of mine really got my attention. I owe it to y'all to come clean, and take whatever lumps I have coming.   

[CoffeeCan]

1 hour ago, donkaye said:

This is a good reason to practice soft field takeoffs.  Mastering this takeoff will teach you how the airplane behaves in the most critical situation and how you SHOULD behave is such a situation.  The airplane will leave the ground before it really wants to fly because of ground effect.  So, it is important to STAY IN GROUND EFFECT until you reach Vx, then, and only then, begin your climb out.  And ground effect means GROUND EFFECT, as close to the ground as possible.  Remember induced drag is reduce by almost 50% when your distance above the ground is 10% of the wing length.  On our 36 foot wing that is 3 feet, and 3 feet is the height of the wing above the ground when stationary.  So STAY LOW!  I have found  that most people in training think they are going to hit the prop and after the takeoff quickly get out of ground effect.  Train yourself to stay low!

Also, high DA's also mean flying a wider pattern to account for the higher true airspeed.

Excellent advice, sir, thanks!

When I did my Mooney transition training a few years ago, my CFI had me work on soft field takeoff technique, but it wasn't really practical at 800' MSL on a 90 F Texas afternoon. I've made a note of your ground effect remarks. 

[MooneyMitch]

1 hour ago, kortopates said:

. This is why on the third day of my transition training course I take the candidate up to a high density altitude airport such as Bg Bear

Indeed Big Bear!  Departing there back in August 2008 in an E model, 2 souls, 1/2 fuel load, OAT 105 F, YIKES!!! 

After coaxing the liftoff, ground effect  was my friend for a long, long way down the runway,  with those HUGE trees  looming ahead at the runways end.  Patience is a virtue.  

Fortunately for me, way back when, my initial flight instructor beat many things into my brain, including situations such as I’ve described.

Soft field t/o’s, density altitude, high altitude airports and leaning correctly, ground effect, etc. are my friends! 
 

Thank you Barney Kuhns ! 

P.S.  Again I say, do make sure your tires are correctly inflated, always!  Under inflated, all those calculations are for not ! 

[Ibra]

Yes as other said what works for soft field also works for soft air  

I am jealous there are runways US where one can fly a Mooney in high altitudes !

Assuming one has done proper calculations and has proper technique, the decision making on high DA takeoff is still tricky than normal takeoff as the gap VR to VY is long: there is no point rotating if you can't accelerate in ground effect to VY for climb, so the decision to reject acceleration in ground effect is way tricky than reject takeoff on the ground

Wondering if that "50/70 rule of thumb" should be adjusted to have VY*2/3 at TOD*1/2 rather than VR*2/3 or VS*2/3? to be more conservative
Also VY does get slightly adjusted for altitude, likely to go down in NA but it seems to stay where it is in Turbos

Between VR & VX/VY, something needs to be done about the gear? some teach gear should stay down while you have remaining runway, but not sure if it's a sound advice at higher DA, wheels will not offer much protection with 200ft remaining runway at 100KTAS, my gut feeling gear should go up ASAP for quick acceleration & better climb, worst case landing with 200ft remaining runway is still possible on the belly if my aircraft is not moving forward 

Edited May 12, 2021 by Ibra

[jaylw314]

3 hours ago, MooneyMitch said:

Indeed Big Bear!  Departing there back in August 2008 in an E model, 2 souls, 1/2 fuel load, OAT 105 F, YIKES!!! 

Holy smokes!  It's a wonder you weren't sinking through the runway! 

[jaylw314]

2 hours ago, Ibra said:

Between VR & VX/VY, something needs to be done about the gear? some teach gear should stay down while you have remaining runway, but not sure if it's a sound advice at higher DA, wheels will not offer much protection with 200ft remaining runway at 100KTAS, my gut feeling gear should go up ASAP for quick acceleration & better climb, worst case landing with 200ft remaining runway is still possible on the belly if my aircraft is not moving forward 

I recall reading somewhere that tests done showed at slow speeds near Vx/Vy, drag from the gear was less than drag from full flaps, hence most checklists and POH list "raise flaps to 1/2" before raising landing gear.  Practically, I'd suspect the difference is pretty small, though

[flyboy0681]

I announced my retirement for next March and one of the first items on my post-retirement bucket list is a transcontinental tour over a period of several weeks. Since I have never flown out of anything higher than sea level, DA is one of my biggest concerns on the trip and the tips here have been pasted into the document I am compiling for the adventure.

[MooneyMitch]

1 hour ago, jaylw314 said:

Holy smokes!  It's a wonder you weren't sinking through the runway! 

It was an extreme situation I believe .  We’d waited several hours for some cooling prior to departure.  Temps did drop to near the 100 mark and off we went .

Some Mooney folks in a C model departed a couple hours prior to us with 3 souls and full tanks. We all seriously thought they were really going to depart the earth, literally! 
 

We gasped as they lumbered and wobbled barely airborne, down the runway,  nearly stalling a few times.

Miraculously staying airborne and staying in ground effect, they somehow managed to have enough airspeed to pull up drastically just prior to impacting the trees.  No sooner had they cleared the trees, we saw the plane drop out of sight on the other side..  Out loud gasps commenced again as we all thought they’d crashed.

Within a few more moments, we saw the plane airborne making a gentle left turn, fortunately following the sinking terrain.  
 

Had they not followed the sinking terrain, I believe they would have definitely crashed.

One of those moments one never forgets.

[201Steve]

I had this experience in Albuquerque and El Paso last year when I took my J coast to coast. Never been much out of 1,000’ elevations and we were at near gross. I was happy to have 8,000 foot runways during the experience. It was hotter than hell in July, I felt like airspeed sat at 60 knots before rotation for an eternity. Plane started flying, lost ground effect accidentally, and sank back down 2’ above the runway. Thought we were going to bounce. The bird held its position and a very uncomfortable shallow climb ensued. DA was prob abt 7-8.  Fun times. 

[cliffy]

My D/C lived at KPGA for 13 years. Most summer days were 7,000+ DAs   One gets used to how the performance feels with experience. Its nothing to fear- just different.

Big Bear is famous for overloaded airplanes due to the cheap gas prices there.  Many many have not made it past the trees there on a hot summer day. They live at sea level in the LAX basin and go to Big Bear to get away and are enticed with the cheap gas. There's a reason its so cheap!  NO ONE IN THEIR RIGHT MIND FILLS UP IN THE SUMMER THERE!

As far as high DA takeoffs are concerned- you haven't lived until you do one at max gross (for the conditions) in a 737-200 with puny -9 engines on a hot summer day in Denver. Tire speed was a limiting factor as our ground speed to get to the indicated airspeed we needed went WAY high. It was a flaps 1 improved T/O IIRC instead of flaps 15. Lose one at takeoff and you were a hood ornament  for 15 miles fighting for altitude. Do it once in the sim and it makes you a real believer. 

NO stones at the OP here because every pilot has done something where "learn by doing and don't do it again" plays a part. Hoover, Yeager and even Bill Wheat (as told to me ) have done it. 

I've posted this before but bears repeating-

One is not a safe pilot until one becomes "tempered". And you can't become "tempered" until you do something in an airplane that scares the shit out of you and you know you did it to yourself. Flying takes on an entire different meaning after that experience.

This may be the OPs temperance flight. It will undoubtedly affect every take off he makes from here on probably for the better. Good job of analysis and recovery.

[Ibra]

2 hours ago, jaylw314 said:

I recall reading somewhere that tests done showed at slow speeds near Vx/Vy, drag from the gear was less than drag from full flaps, hence most checklists and POH list "raise flaps to 1/2" before raising landing gear.  Practically, I'd suspect the difference is pretty small, though

Yes on M20J, drag from gear is not much bellow 80kts but it may make acceleration with limited power from 50kts to 80kts very painful 

I wondered if drop gear & 1/2 flap at 20kft brings one down to same ceiling as climbing with gear & 1/2 flap down from 6kft?

[Will.iam]

1 hour ago, Ibra said:

Yes on M20J, drag from gear is not much bellow 80kts but it may make acceleration with limited power from 50kts to 80kts very painful 

I wondered if drop gear & 1/2 flap at 20kft brings one down to same ceiling as climbing with gear & 1/2 flap down from 6kft?

You can never climb as as high as you can descend. 

[Will.iam]

2 hours ago, cliffy said:

My D/C lived at KPGA for 13 years. Most summer days were 7,000+ DAs   One gets used to how the performance feels with experience. Its nothing to fear- just different.

Big Bear is famous for overloaded airplanes due to the cheap gas prices there.  Many many have not made it past the trees there on a hot summer day. They live at sea level in the LAX basin and go to Big Bear to get away and are enticed with the cheap gas. There's a reason its so cheap!  NO ONE IN THEIR RIGHT MIND FILLS UP IN THE SUMMER THERE!

As far as high DA takeoffs are concerned- you haven't lived until you do one at max gross (for the conditions) in a 737-200 with puny -9 engines on a hot summer day in Denver. Tire speed was a limiting factor as our ground speed to get to the indicated airspeed we needed went WAY high. It was a flaps 1 improved T/O IIRC instead of flaps 15. Lose one at takeoff and you were a hood ornament  for 15 miles fighting for altitude. Do it once in the sim and it makes you a real believer. 

NO stones at the OP here because every pilot has done something where "learn by doing and don't do it again" plays a part. Hoover, Yeager and even Bill Wheat (as told to me ) have done it. 

I've posted this before but bears repeating-

One is not a safe pilot until one becomes "tempered". And you can't become "tempered" until you do something in an airplane that scares the shit out of you and you know you did it to yourself. Flying takes on an entire different meaning after that experience.

This may be the OPs temperance flight. It will undoubtedly affect every take off he makes from here on probably for the better. Good job of analysis and recovery.

Try a tanker with A model engines that were injecting water for added thrust but you only have 90 seconds before the water runs out and you better have sucked up the gear and flaps by then or your not going to make it. Also at 175 knot rotate speed you eat up 1000 feet before the mains liftoff so even if you are not quite at Vr if the 1000ft remaining board is wizzing by you start rotating or the gear is going to take out approach lights! Heard of storied of guys praying they make it over the perimeter fence. On some higher DA fields we were limited to how much fuel we could take to keep from exceeding max tire rotation speed of 195! With no thrust reversers on the R model your V1 speed that allows you to stop is very low (saw 115 a few times) on max weight takeoffs and VR is very high (165+) so you have this loooong time after you are committed to going that even if you blow an engine you are still going with serious seat cushion suction and it seems to take forever for the speed to increase all while the end of the runway when you started was over 2 miles away but now is rapidly coming at you getting bigger by the second. I don’t miss those days at all.  

[Andy95W]

5 hours ago, flyboy0681 said:

 DA is one of my biggest concerns on the trip and the tips here have been pasted into the document I am compiling for the adventure.

One of the best quick tips I’ve read here on MooneySpace was from a few months ago.

When the density altitude is greater than the runway length, stop and work the numbers and make sure you’ve reviewed your plan.