50/70 Rule (acceleration abnormal check) • An EFATO survival story.

[Garfly]

Dan Gryder's talk with a pilot whose perfect P-chart calcs let him down due to a sick, underperforming, engine:

 

 

[facthunter]

That's NO revelation. That the motor(s) have to be making the right power for the "P" charts to mean anything. A Power check should be part of every take off. Many f those critical speeds are actually representing a point on the runway.   Nev

[Garfly]

21 minutes ago, facthunter said:

 A Power check should be part of every take off. 

Very TRUE  (and closer to education than revelation ).

Though Gryder and the (800 hour) aviator involved were at pains to explain that such a check had not been part of his (the incident pilot's) education heretofore. 

Revelatory to him, at least.

 

 

 

Edited October 1, 2022 by Garfly

[facthunter]

The best way would be to use an accelerometer with  the right program and the figures fed into it. Not only small aeroplanes get  caught by this problem . It can potentially kill a lot of people.  A dragging brake can have a similar result. or having too much flap set. Nev

[Yenn]

An accelerator is just another thing to worry about. How about checking for revs at the start of the takeoff. I know I should see about 2700rpm at the start of the roll. If I don't see that something is causing the engine to lack power.

[facthunter]

IF you have a CS prop it won't vary. Nev

[Thruster88]

A stopped or broken engine has the highest manifold pressure (turbocharged excepted). Constant speed propellers mask engine problems. 

[Garfly]

I suppose the most available abnormal-acceleration check is our own sense of what normal feels like.

But we need to listen to the inner "something's wrong" voice - and be ready to reject in good time.

But with all the variables at play in take-off performance, our sense of normal can be confused (assuming all due P-chart compliance) especially operating away from home. I suppose that's why the 50/70 Rule (70% of take-off speed by 50% of runway length) was invented.

As a rule of thumb, it accounts for most of those variables (variations in slope and/or surface along the run excluded). 

Of course, it means the PIC has to mark (mentally, at least) the midway point in advance and have the 70% number in mind.

Seems like it'd be worth experimenting with.  Anyway, old Gryder swears by it ... and the pilot in the video is a keen, if recent, convert. 

 

 

 

 

Edited October 1, 2022 by Garfly

[aro]

What normal feels like isn't always accurate. e.g. if you normally fly solo out of Moorabbin, a takeoff at Mt Hotham with a full load will feel a long way from normal. The performance charts tell you whether you can do it.

 

But I think the video misses the main point. In a single engine aircraft, if the engine isn't producing enough power to fly you are better off rejecting the take off at any point prior to lift off. Earlier is obviously better, but getting off the ground will practically never improve the situation.

 

The distances they described sounded like he did actually have enough distance to stop if he had made that decision. A couple of hundred metres should be plenty from <60 knots when you are already on the ground.

 

50/70 is more of a double check of the performance calculations than a power check. If the engine produces consistent low power from the beginning of the takeoff, it probably works. It doesn't necessarily work if engine loses power part way through the takeoff.

[Garfly]

1 hour ago, aro said:

What normal feels like isn't always accurate. e.g. if you normally fly solo out of Moorabbin, a takeoff at Mt Hotham with a full load will feel a long way from normal. The performance charts tell you whether you can do it.

Agreed ... or as I'd put it:  "with all the variables at play in take-off performance, our sense of normal can be confused ..." 

And, to paraphrase Nev, with P-charts, it's garbage-in/garbage-out.  To me that says a backstop rule is handy.

 

1 hour ago, aro said:

But I think the video misses the main point. In a single engine aircraft, if the engine isn't producing enough power to fly you are better off rejecting the take off at any point prior to lift off. Earlier is obviously better, but getting off the ground will practically never improve the situation.

True, but I don't think the video really misses that point.  Gryder stresses that a sick engine is only one of several possible causes of unexpected poor acceleration and his 50/70 rule of thumb is meant to catch them all.  And, as you say (and the video, too), it's only good as a last line of defence.  Nobody denies that stopping immediately anything seems wrong is the way to (not) go.

And sure, in the real world, like taking off at Mt. Hotham on a hot summer's day (P-calcs duly done), having a backstop rule-of-thumb to counter "inaccurate feelings" might help.  Even - turning it the other way - to prevent a dangerous reject situation which was never, in fact, necessary.  (Visions of a cliff-hanger ;- )

 

Also, you might say that this video demonstrated a case where pulling it off the ground may well have improved the situation. According to the pilot's story, the alternative could have been plowing into the school pick-up event unfolding in front of him. Of course, he's the first to agree that an early reject was what was called for - if only he'd learned 50/70 earlier, he reckons.

 

But yes,, as Dan G says in the video, actually living to tell such a tale is not at all common.

 

 

 

 

 

 

Edited October 2, 2022 by Garfly

[greybeard]

On 01/10/2022 at 1:56 PM, facthunter said:

IF you have a CS prop it won't vary. Nev

Wouldn't the manifold pressure give it away? Been a few years since I did my CSU but I seem to recall that was the goto power check instrument.

[Bruce Tuncks]

While appreciating the constant speed prop stuff, for me, with a fixed pitch prop,   I like Yenn's idea that if you are not seeing the proper rpm then you have a problem and should abort.

 

[facthunter]

GB mostly for a boosted engine you check rpm and MP. It's generally accepted  the governor will mask the loss of power. You'd need a torquemeter which is more common with Turboprops.

 For Bruce if you doubt the engine, fix it, but at least do a static rpm check on the brakes, prior to rolling.  Nev

[walrus]

Yes, a constant speed prop may turn at specified rpm. It may also indicate full power MP and be sick as a dog. That is why you MUST  check That you have full power FUEL FLOW.

 

Revs + MP +Fuel Flow are needed to confirm full power.

[aro]

Revs + MP + fuel flow still don't guarantee full power for a constant speed prop e.g. if the problem is ignition related.

 

Also, if you are taking off at a high altitude airport you have a reduced MP and probably need a reduced fuel flow (i.e. leaned) for maximum power.

[Thruster88]

Yes the "fuel flow" gauge in an IO-xxxx Lycon engine aircraft is just a pressure gauge after all. Normally x amount of pressure results in a known calculated fuel flow, a partial or complete blockage in a fuel nozzle will upset the apple cart.

[Garfly]

Juan Browne on the 50/70 rule: 

 

 

[facthunter]

 50/70 only holds true if the plane is still accelerating significantly With the tail still low that might not be happening. That plane is underpowered and at that altitude had really no chance of flying away. Nev

[Garfly]

28 minutes ago, facthunter said:

 50/70 only holds true if the plane is still accelerating significantly With the tail still low that might not be happening. That plane is underpowered and at that altitude had really no chance of flying away. Nev

 

Yeah, I don't think it'd apply particularly well in this case.  The uneven slope and variable surface of the 'runway', alone, would make it unreliable.

But still, just having any such performance/acceleration check in mind - on any unusual take-off - ought to give pause enough, you'd think.

If I was ever so adventurous as that chap, I'd try a solo take-off first and then assess if any more load was feasible and safe.

Maybe helped by another rule-of-thumb:  For a given situation, for every 10% increase in T.O weight you need 20% more T.O distance. 

 

I'm not sure how reliable that one is, either, but one could confirm it, for one's own a/c, experimentally by taking note of two max performance take-off runs (in identical conditions) one with, and one without a load - an instructor, say.  The numbers could be extrapolated to roughly prove (or not) the rule, no?

 

 

 

 

 

 

 

 

[facthunter]

It could also be the case that the plane is at or near it's service or absolute ceiling already. Nev

[Garfly]

Yeah, you'd be wise to abandon any testing above, say, 10K DA.

[facthunter]

Following the previous plane but below its path is not smart either. He had no option but to be below  but the wash may have been a factor.  Nev

[Garfly]

 

This is from a 2016 issue of Plane and Pilot:

 

Top 10 Rules Of Thumb

Piloting an aircraft requires decision and precision. Quick references to the basics can make both easier.

 

https://www.planeandpilotmag.com/article/top-10-rules-of-thumb/#:~:text=If you haven't heard,an abort is in order.

[facthunter]

 Situational awareness covers it. What do I have going for me here and what are the risks involved? Assess ahead rather than have to respond to something you  created that is adverse. Prioritise the IMPORTANT things. Ignore the dross and distractions. Nev

[Garfly]

Just came across this other Juan Browne cautionary tale about a rejected take-off :