PilotEdge Forums

rtataryn wrote:In the previous video he had discussed going to Big Bear for the lower fuel costs, so I assume he filled up - possibly to max gross, and that potentially plays into the lack of climb rate too.

Regarding HOW much fuel was on board, in the video they stated the re-fueling took into account the needed take off performance. One would assume that the airports's altitude and expected take off weight were part of that calculation.

rtataryn wrote:Here's a video of a guy that should have done exactly what Mr. Aviation did . . .

^^In this case it was well established the airplane was over weight for the density altitude. The pilot ignored the symptom when on the initial take off roll the airplane initially went airborne but settled back down. IMO the accident sequence was sealed when he elected to continue down the runway.

If you look carefully at the video you can see the pilot pulling up on the nose followed by the airplane slowing down and the nose settling back down. On other video taken from the side, the high angle of attack between the wing and horizon can be seen. Clearly the airplane didn't have the performance to climb under the situation.

Please note, my question/point was not about the decision to abort. Clearly the airplane didn't climb as the pilot EXPECTED it to. During the takeoff roll isn't the time for extensive trouble shooting.

My question was more about why the airplane didn't climb as expected. I had doubts that the tailwind was the only cause.

I also didn't understand the reason why the pilot choose to perform a short field take off when there was more than enough runway. The follow on question is if the short field take off could have played a part in the sequence.

What would have been the likely outcome if a no flaps take off was done?

inquiring minds want to know

wmburns wrote:Regarding HOW much fuel was on board, in the video they stated the re-fueling took into account the needed take off performance. One would assume that the airports's altitude and expected take off weight were part of that calculation.

These are important discussion points for this aborted takeoff and would have been nice to have seen clarified in the video. What was the takeoff weight? What was the density altitude? What does the POH state as the maximum density altitude for takeoff? How was the weight adjusted to compensate for the DA? Without any discussion of these factors in the video, one wonders if any of these calculations were actually performed.

wmburns wrote:rtataryn wrote:
Here's a video of a guy that should have done exactly what Mr. Aviation did . . .

^^In this case it was well established the airplane was over weight for the density altitude.

True, but it was not overweight to fly within it's DA envelope. The 165 hp Stinson was 2314 lbs at takeoff with a 2400 max gross takeoff weight. As stated, the problem was the DA. The DA was 9167 ft, and the takeoff performance charts for the Stinson max out at 6000 ft DA. Dumping more weight and hoping the engine and airframe can still climb in a DA that is over book specs is test pilot stuff.

wmburns wrote:My question was more about why the airplane didn't climb as expected. I had doubts that the tailwind was the only cause.

Short of a microburst downdraft, the direction of movement of an airmass has no bearing whatsoever on climb performance once an aircraft is airborne. Aircraft weight and Density Altitude are the elephants in the room in this video.

Field elevation: 6752, temp 5, dew point -2, density alt: 7267.

I'd be stunned if a Skyhawk with 2 people on board tops out at 7000ft. Just checked a 1981 172P POH (close enough). Assuming it's leaned for max power and is at max gross, distance to clear a 50ft obstacle is around 3200ft (zero wind). Add 10% for each 2kts of tailwind, up to 10kts.

In terms of climb rate, FLAPS UP, max gross (worst case) at 7000ft at 5 degrees C should yield 365ft/min at 72kias. They were well short of that speed, though.

While steady state tail winds don't hurt climb rate (just the climb gradient), gusty tailwinds or a rapidly stiffening tailwind can be bad news if you're already in a high alpha situation (as they were). They needed everything to go just right to get 365ft/min, and they were 10kts short of the required speed to achieve that climb. They were behind power curve and the gusty tailwinds were causing momentary losses in indicated airspeed, resulting in a higher angle of attack to maintain altitude. Given an infinite amount of room, they would've eventually got it 'on step' and out of the high alpha situation, but short of that, I don't have a good sense of how much longer it would've taken to get everything squared away.

Note: regarding fuel, if they were indeed going to Salinas (KSNS), 274nm away, I have to imagine they'd have full tanks (~40 gallons). Again, a no wind takeoff shows 3200ft to clear a 50ft obstacle. With 5800ft available, I would not have been concerned about the takeoff roll distance in terms of performance planning. However, the 365ft/min climb rate is an indication that there is little excess power available and that it isn't going to take much to make life difficult.

Flaps 10, insufficient speed and the possibility of incorrect mixture (I saw it was leaned, but I have no idea if they nailed it for best power) thwarted them in the climb.

Great discussion Keith. With those numbers it does make one wonder about your earlier point regarding the mixture setting and the engine producing optimum power. Also, I wonder with the mountain ridge to the south and a 160 degree wind, if there could have been a downdraft or two coming off those 10,000 ft hills.

This is from the comments:

"The decision to roll with flaps mainly came from the idea of performing a short field takeoff on order to fly out of ground effect (gain altitude from the runway) sooner. The flaps at 10 would have allowed us to hop off the ground sooner and would have a little better climb rate at Vx, so make SURE we cleared the trees at the end and had enough space from the terrain, however, that obviously didn't all work out because the winds shifted."

Now, granted I am a student pilot, but in my limited experience with short-field take offs, I found the use of 10 degrees flap marginal at best on the 172M. If anything, I think it contributes to a better Vx, but I can't say I have seen a huge impact on the time it take to get the wheels off the ground. But I can see what he was saying, get off the ground as soon as possible to gain the best airspeed without the added friction of the tires on the pavement.

Again, low-time student pilot here, I could be way off in my analysis.