Discuss the problems and solutions to all of the situations that Pilot X finds himself in.
#1000404
Three Big Bears

Richard Boswell follows three experienced pilots towards a very small gap in a line of very tall trees…

From the relative comfort of his airline seat, X peered out of the window at the rugged mountains below. As the 747 powered skywards, X identified the barren ski runs and made out the dry lakebed. From there it was easy to pick out the runway and X could just about make out the line of trees that had come so close to being the cause of his demise.
He turned to his colleague and smiled. “Those conifers don’t look too bad from here. Want to go back and try again?”
“Yep, what could possibly go wrong?” was the sarcastic reply.
The pilots were making their way home after a week’s course in California – a week spent in a darkened classroom, making notes and concentrating hard. The course concluded on a Thursday and their return flight to Heathrow didn’t depart until late in the afternoon the next day. Not wishing to miss a valuable opportunity to go flying,
X booked in to do his biennial flight review at a local flight school – and with a brand new 172 available for less than £100 an hour – with an instructor – it seemed churlish to get airborne and merely burn holes in the sky. He would combine it with a Big Bear City checkout. This tarmac-runwayed airport, an hour’s flying from LA, was at 6,500ft altitude – a gateway to the ski slopes just beyond it.
The plan was beautiful in its simplicity. Being a 172, all three would make the journey. X’s colleague sat in the back with his camera bag, and they carried only half-a-tank of fuel to give a little spare weight for the take-off at altitude for the return leg. The little Cessna climbed out over LA, slotting into a flight-following service that made flying around some of the world’s most congested airspace simplicity itself. The views over the sprawling city were spectacular as the instructor gently quizzed X on the few local rules and limits.
After 45 minutes, conurbation turned abruptly into mountains, the wonderful geography that allows you to surf in the morning and ski in the afternoon. GPS aided the navigation and the new Cessna, with X at the controls, flew a visual approach into the sunshine of the quiet airport. Lunch followed, and the three pilots discussed
the merits of the different options open to them for the departure. A downwind take-off would give them an 8kt tailwind, but would allow for an uninterrupted departure over Big Bear Lake. An into-wind take-off meant that they would need to be sure that there was enough grunt in the engine to clear the row of tall trees which guarded the upwind end of the runway.
Over a cheeseburger and POH, it was decided that there was no future in downwind departures at altitude, especially as the OAT was now pushing 30°C and the QNH was just 29in. With three burger-fed, slightly rotund males, the book said they needed 4,500ft to clear 50ft. They had 5,500ft of tarmac available to them. The decision was made.
At the end of the runway, the board gave as many clues as you could need: ‘PILOTS MUST ENSURE THE AIRCRAFT IS PROPERLY LEANED
FOR TAKE-OFF’.
X couldn’t miss it, and as he opened the throttle to 1,800rpm for the run-up, the engine coughed and spluttered by way of a reminder. He had already wound the mixture out a little after start to keep the engine running sweet during the taxi, and now winding it out still further, the rpm climbed to 1,900 and the engine sounded smooth. X wound the mixture out still further and the engine started to run rough again. He wound it back in until the engine hit the sweet spot again, and he checked the mags, 10° of flap selected and everything seemed to be working just as advertised.
But X took no chances: he positioned the aircraft on the threshold so that there wasn’t a spare inch of runway behind him and held it on the brakes as he opened the throttle. At full power the engine was barely producing 2,000rpm. With the brakes released, it started to roll forward slowly and already the trees at the end of the runway were starting to look big.
The speed built up slowly and with it the rpm incresed a little. Everything seemed to be happening in slow motion. The only thing that appeared to be in real time was the amount of runway being used. At 60kt, X gently eased the aircraft off the tarmac and with the stall-warner loud in his ear the aircraft continued in level flight barely 20ft above the ground, and towards the conifers. Each gust of wind knocked a few knots off the airspeed and brought the aircraft closer to the stall. The book had said the aircraft could do it but blatantly it couldn’t, and now there wasn’t enough runway left to stop the aircraft before the hazard.
A silence descended on the cockpit as both pilots sought inspiration. Just to the left of the nose was a small gap in the trees, perhaps just big enough to squeeze a struggling 172 through. It was the only option that they had open to them. With the wingtips just feet from the trees, the aircraft struggled through towards the relative safety of the dry lakebed beyond.
Now at least they had options, and one of them was to attempt to get the struggling Cessna to gain altitude over the lake, but putting in only 10° of bank killed any hope of a climb. Another option was to land, either on the lakebed or to attempt to go back through the trees onto the runway.
The third option was to struggle over a saddle in the mountain ridgeline; it was clear beyond, with the ground dropping away almost 3,000ft. The feeling of elation from the three pilots was palpable as they realised that good fortune had saved them from one of the most fundamental errors. ■

1 Why didn’t the aircraft perform as advertised?
2 Did X make the correct decision with regards to the take-off direction?
3 What was the density altitude for the airport on that day?
User avatar
By Keef
#1000466
Big Bear City is a delightful airfield, but it's in a region where density altitude needs to be taken very seriously. Most AWOS stations around there either announce "Caution! Density Altitude!" or actually quote the DA as part of the information.

In this case, it's hot and the pressure is low - both bad news for the pilot: the DA is somewhere around 10,500 to 11,000 feet (depending on the dewpoint, which is not stated). At that DA, C172s aren't known for a sprightly rate of climb. I would be reluctant to take one, three up (even if light on fuel) into or out of Big Bear in those conditions.
By mixsfour
#1000554
The main reason the plane didn't perform 'as advertised' was incorrect leaning procedure. The POH states the mixture should be adjusted to give highest revs at full throttle, not from a starting point of 1800 rpm. The low max static revs was a clue

Another reason was use of flap. POH does not recommend use of flap for take-off at high altitudes.

The correct runway was used. The tailwind component on the other one would increase the ground run dramatically, in this case in excess of 20%

Density Altitude was around 11,000 ft at the time
By masterofnone
#1000922
The first two responses probably have it nailed. The only other thing I can think of which may cause the aircraft to not perform as advertised is due to the brand new engine being "tight" (ie not completely run in) and so not developing 100% power relative to the POH assumptions.

If like me you have no ide what this place looks like - here's a youtube link showing what it's like approaching the airport
Last edited by masterofnone on Fri Oct 14, 2011 2:07 pm, edited 1 time in total.
User avatar
By Winhern
#1001067
'Took off at 60kts with the stall warning blaring'

Sounds like the classic case of rotating too early due to worry about the trees and thus having a very poor rate of climb. :idea:
User avatar
By kanga
#1001181
Winhern wrote:'Took off at 60kts with the stall warning blaring'

Sounds like the classic case of rotating too early due to worry about the trees and thus having a very poor rate of climb. :idea:


At a short strip towards an obstacle, I have occasionally wondered when it is worth accelerating in ground effect once airborne rather than going straight for POH best angle of climb speed. I supect that I may not always have made the better decision! I recall an almost calm day, warm, at Popham, with 21 in use (ie towards the trees) by local microlights; in retrospect I'd have done better using 03 instead. But at a significantly higher DA, I'd be inclined to wonder whether the poor power output would provide the necessary acceleration.

However, with no upwind obstacle, I've used the acceleration IGE very effectively; eg, 27 at Croft Farm on a warm calm day. I gather it alarmed some spectators, though!