POH Vs The Real World

[Guest ozzie]

came across an interesting story the other day about an aircraft accident that involved some inconsistancies between the performance figures in the Pilots Operating Handbook and the actual performance of the aircaft.

 

During our training we are introduced to some advanced techniques for things like short field and high density takeoff procedures that involve the need to consult the POH for performance figures and charts. Most pilots rarely if ever practice these skills on runways of adequate lengths to familarize themselves after their initial training and only refer to the POH if they are operating from a short and or high strip.

 

Here's my point. You are invited to a friends strip, your flying a 1975 model C172 one pax half fuel. it's not to hot( 22 c) and the strip is around 800ft asl. you have lunch and then decide to leave and hop in and line up. you have had this aircraft for a couple of years and have basically flown in and out of your home airport so you have a quick look at the performance figures and chart for the departure from the 350mt strip that has a row of 50ft trees near the end. Ok so you set 10 of flap bring up the power release the brakes and way you go. initial acceleration is good but seems to drop off a bit as you get to your lift off speed but you are committed now and you barley get out and clear those trees. what happened the POH said that you should have got out of their without that size 3 grommet factor. so where is the aircraft's performance. the engine is running right power check was as it should be it is only 300 hrs since it's last overhaul.

 

So you guys tell me why that aircraft did not perform as the POH said it would.

 

List the reasons that could affect the take off and climb and other factors like fuel burn stall speeds ect. Then go and have a look at your aircrafts POH and compare your aircraft to the list you have just made and see how it could affect you under tight conditions

 

then go and actually compare those figures in the POH and what your aircraft actually does on a long obstacle free runway before you try it in a real situation and suddenly find yourself on the backside of the curve slowly sinking into the trees.

 

 

[turboplanner]

Depends whether the only checks made were those you mentioned, or whether a full set of calculations was done. If it was the former, then the result was understandable. If it wasn't, I'd put my money on the "friend's strip" increasing the rolling resistance.

 

 

[Guest Cloudsuck]

I agree with Turbo. What was the strip like? Contributing factors could be:

 

 

- Strip grass length (how tall is the grass)

 

- Strip grass wet

 

- Strip slope

 

- Wind strength and direction.

 

 

For a start, if you have a 350m strip with 50ft trees at one end, your TODA is not 350m.

 

 

If you have any sort of head wind, you could be taking off into lee side rotor given off by the trees at the end of the runway and as you approach your lift off speed, you hit the dead/down going air. Remember rotor can extend for up to 10 times the horizontal distance of the vertical height of the obstruction (e.g. 500 feet or approx 160m).

 

 

The biggest mistake in this scenario was choosing a 172.

 

 

[Guest ozzie]

Ok it is just your average level dirt strip. this is aircrafts actual performance vs the POH figures' wind is light the day is average nothing to really make you pull out the slid rule.

 

aircraft is 30 years old, two private owners with about 2500hrs on frame, engine 300hrs into it's second life, prop is half life, half fuel, one pax. run up showed ops normal. wind light 0/5kts on the nose. temp 22, strip ht 500ft asl the 50ft trees are 75mts beyond the threshold. road between them and strip. pilots experience 1000 hrs plus. we will say here that aircraft cleared the trees by 10ft when the book says it should have been 100ft.

 

initial acceleration was normal but seemed to stagnate a little towards the rotation point but engine instruments showed normal. being a C172 has no meaning on this it could happen to any type.

 

what went on here ?

 

 

[Guest Decca]

It's a 34yr old aircraft, has it been cut & polished lately? I need to check, but I think there is a small consideration in the Operating Handbook for performance deterioration allowed in the figures.

 

The temp is "only" ISA +8. But that adds another 900' to the density altitude.

 

And what's the QNH ozzie? Any decrease below ISA pressure is going to increase pressure altitude, decrease performance.

 

Best rate of climb will increase clearance margins. All this and other post contributions need to be factored in for potentially marginal performance.

 

Regards, Decca.

 

 

[turboplanner]

Ozzie, from your second post, I think it is a P&O issue, the pilot reaction almost confirms it, and even 1000 hour pilots can miss something as the accident reports show.

 

For what it's worth, I've been in the situation where the nose was staying below the tree tops, and I know how time slows down.

 

Haven't got time to spend on it right now, but I'm overdue to put out a couple of P&O exercises where there may be some ingredient you could point to.

 

 

[poteroo]

A few questions.....

 

Is the tacho accurate? Even 100 RPM out makes a big difference

 

Is the propellor the right pitch? Has it been changed at o/h, noted in the prop logbook.....which of course pilots don't read very often.

 

Whats the propellor condition like? Pitting of the rear face by metal dust is a real negative on performance...but most pilots only look at the leading edge.

 

Did the pilot see the full static RPM as required for this pitch propellor? Does the pilot even know what the static rpm should have been?

 

Did the pilot make a rolling start - or a brakes on at lineup, (=textbook),start. With a rolling start you'll be 10 kts ahead of Capt Textbook.

 

!0 deg of flap probably isn't enough either. With a light load, 20 would get you into a BAOC much sooner.

 

Did yr pilot angle the t/o run so as to maximise the h/w and minimise the x/w? Did he turn upwind immed after liftoff so as to allow more distance to the trees?

 

Just a few possibilities that come to mind.

 

happy days,

 

 

[Yenn]

Looks like a prop problem to me. The acceleration fell off approaching rotate speed, but I experienced a similar thing years ago. the run up was fine and it reached static revs OK, but it would not climb. Took 20 mins to get to 5000'. When I took it back to the owner I said it was sick and noted in the maintenance release. The owner took it to Rocky for a LAME to check and it was pronounced sound. Somebody else hired it with the same outcome and it went back to the LAME at Rocky, again pronounced sound, but this time it had an engine failure at take off, safely landed on the runway. Verdict 2 exhaust valves burnt.

 

 

[Guest ozzie]

OK some things that were pointed out during the story

 

obtaining the figures for the POH during certification of the Aircraft. the obvious is a brand spanking new aircraft. the other is the test pilots, highly experienced very capable of flying right to the performance of the aircraft.

 

other things to be taken into consideration was the overhauled engine. They never do as well as a brand new unit plus they seem to run best towards their time ex. the prop was not quite as efficient as when it was new. filed out a few to many stone chips maybe.

 

again the paint work not as clean ect. now the tires, pressure and temp could change during the run and maybe the brand had been changed and gave a bit more rolling resistance. Was the oil a full operating temp? how about the fuel? does it produce as much bang as it did 30 yrs ago. did the pilot track straight down the strip or a little bit of "s" turning with a bit of drag from the control surface application to keep it straight. Add a few other little things and that was enough to slow the acceleration thru the rotation and initial part of the climb out to keep him on the back of the power lift curve.

 

I will try and find the link to this story when i get home but i can't remember where it came from (40mb download.) In the story the aircraft was a cardinal and they went into the trees after a bit of dodging and turning and getting futher behind the curve

 

The point was and is what i am making here. Are the figures and charts in your POH really applicable to your aircraft and if you follow them with out allowance like most pilots do will it eventually get you into trouble. The recommendation was to go out and set up for short field work on a long runway and using the POH see just how much error there is. May save your neck one day.

 

 

[turboplanner]

Ozzie, you're talking about a much bigger error factor than those little things.

 

Are you going to practice in the Lazair?

 

 

[Guest Ken deVos]

C177 is not a C172

 

I know this is leading the thread astray, but a C172 is not a C177. From the stories I've heard, the early 177s had a real issue getting on the right side of the power curve.

 

A friend who owned a 177 in the 'good old days' had taken a group of pax into the outback. Unbeknown to the pilot, one of the pax was a rock collector and at each stop would squirrel away a sample for his collection.

 

Long story short, on a short dusty strip the 177 refused to climb. The aircraft was pulled over the fence at the end of the strip, only to descend into the dam on the other side.

 

Luckily no injuries except pride, especially when the property owner revealed that they usually 'drop the fence' for their aerial operations.

 

As for the aircraft, DZI still flies and was last seen at YMMB.

 

 

 

[Guest drizzt1978]

Was the passenger like one of those 250kg guys, cause thats really gunna stuff up the calculations....

 

 

[Guest ozzie]

I was just using the C172 as my example to keep it simple. The "gutless strutless" cutless as they were called had some real issues with poor power and a strange type of laminer wing secton. you did not retract the gear until you had a real good rate of climb.

 

what i am talking about it IE differencs between the POH figures and the actual performance is applicable to any and all aircraft.

 

As for the Lazair it has a rate of climb of 150fpm on a good day. So as you can guess i've had some fun climbing out on a hot summers day.

 

Much the same for my old Condor and Stolaero at the old St Mary's strip i was the only one who could just manage a circuit on a summers afternoon only because i weighed in at 55kg then. The others heavier guys only got airbone because they were going downhill but on the base leg had to do a fast taxy back to the strip, playing around tying to fly a few MPH above the stall teaches you how to be gentle and coax the aircraft. Minimum aircraft was the game.

 

Had an interesting lesson with the Twin Otter once in plus 40deg at Yarrawonga once.

 

 

[turboplanner]

The principles are the same, but once you start cross comparing or using rules of thumb, the leverages start to have a major effect on the result. The only way to do it, is do the whole calculation based on actuals.

 

I'm still flat out on another project, have to make the sample aircraft fly if you get the answer right and fall out of the sky if you don't and it's hard to design one as close to the wall as that, but I'll put some more time in shortly, then you should be able to isolate what the problem was.

 

 

[Guest ozzie]

i belive that as this guys story showed, is that the older the aircraft becomes and the more things change as the years roll on, like fuel types ect, that the peformance and operating figures in the POH become less reliable as accurate reference points.

 

It may be that in the near future with more of the older GA aircraft like the C150 and such finding there way into Rec aviation we may see a rash of accidents that have been caused by pilots referring to the POH and operating right to the limit of them only to find that the aircraft is not performing to those figures. It may just be a wise choice to go out and actually see what the aircraft is performing like TODAY under controlled conditions and comparing them to the 30 yr OLD POH references and making some reassements to those POH figures. rather than just rock out to some mate's airstrip thinking that you are OK when in fact you are not.

 

 

[Guest ozzie]

Surviving the Crash

 

have a look at this site

 

 

[Guest Maj Millard]

Oz, Did all my initial PPL flight training in C152s and then C172s. My instructor was a stickler for practising everything in the book. Did many short/soft field takeoffs and landings. The works many times over, until I actually enjoyed doing something other than the normal landing/takeoff.

 

All done by the POH book numbers, which I always found to be pretty much right on in the Cessna. Did find however that no two C 172s were exactly alike, which was surprising considering they were all the same model (N or Ms). Sometimes there was a good 5 kts difference between individual aircraft. We had one rental aircraft that had numerous landing incidences over a long period. After repairing the nose gear/ firewall for the third time, we took it up to find out why with a couple of different pilots on board. We decided that it just wanted to fly and land 5 kts faster than the rest for some reason.

 

Than you've got the other end of the stick, where the 'pig' as it was known was flat out ever getting over 100 kts, even on a good day. No two are the same, I suspect you just got a 'pig' that day............................................................

 

 

[Guest ozzie]

yep. no two the same and they keep changing as they get older.

 

Major.you find the DVD in the mail?

 

 

[Guest Maj Millard]

Yes Ozzie got the DVD ok, and had a quick look last week. Will look again tomorrow, and yes that reserve in the photo on the wall sure looks very similiar to our design. I'll have to do a bit of googgling on reserves now to see what I can find. Thanks mate....................................................

 

 

[FlyBye]

Maybe this 172 needs to go back over the scales. Older aircraft get lots of extras tacked on over the years but maybe not into the log book.

 

 

[Guest Maj Millard]

FlyBye, you could well be correct there, plus a bit of deterioration of the external finish, on what is really a medium performance aircraft at best anyway. We once found a couple of kilos of buffle grass seed in the tail behind the rear bulkhead, in a C 172. Appears in a former life she had been used for seeding from the small rear cargo door, and a lot ended up in the tail !!..................................................

 

 

[Guest ozzie]

Makes me wonder what some mait. facilities actually do at a major/annual inspection.

 

 

[Guest Maj Millard]

No comment Ozzie, but you'd be surprised what you find sometimes when you take a real GOOD look !!..........to some maintenance is just costly inconvienance......................................................................

 

 

[Guest ozzie]

I've done plenty of 100hrls and major inspections and they were all done by the book book. i was always amazed at the condition of the aircraft and bookwork when they came to us rarely did anything tally up. left us with heaps of work to sort it all out