CAO 20.7.1B First segment
 

For a turbine twin in the first segment 20.7.1b calls for positive gradient. If the runway has a .2% slope up, does that mean the first segment must actually be .2% to be considered as positive? If the runway slopes .2%down slope would a descending (relative to absolute altitude but not to the ground) flight path of -.1% be considered positive?

My take on it is you can take it as "not negative" so you'll need to be able to stay in ground effect while accelerating to your climb speed while in the takeoff configuration with the critical engine inoperative.

No, the slope of the runway is not relevant.

The WAT limit requirements are for still air and in isolation to other considerations - the relevant bit of 20.7.1B specifically cites no ground effect for the first segment. That is to say, WAT limits are just lines in the sand to give the aircraft an absolute basic level of performance so that we don't have to rely too much on the curvature of the earth ...

If you have an aircraft with a poor first segment then it is sensible risk management to keep the first segment not less than an upslope runway for obvious reasons.

When does the first segment present a problem ? - ie when are you likely to be through screen before the gear has had time to retract ? -

(a) when the performance is comparatively high - low gross weight, low Hp, low OAT. Conversely, for most aircraft, high gross, high Hp, high OAT and you generally don't see a first segment as the gear is up prior to screen. That is, for most aircraft, the runway slope isn't going to present too much of a concern.

(b) high wing, physically large aircraft with nacelle mounted gear which takes forever to retract. A good example is the AW650 Argosy (remember IPEC Aviation ?). The good old Queen of the Skies ALWAYS had a first segment for EVERY takeoff. For such aircraft, first segment is something to consider due to its not inconsiderable length over the runway.

Going back a little while, I looked after IPEC's performance on the AW650 and DC9. For the Argosy, we took a management decision and made it policy to limit the RTOW to achieve not less than runway upslope for the first segment. Lost a few kilos but everyone slept a little more peacefully at night ...

Agree.

Simply - a positive gradient is when the altimeter is increasing .

Dodo - if the runway is sloping up, your altimeter will rise, even though you are still on the ground. The gradient must start at 35ft above the ground.

Once you lift off, a gradient is distance forward vs distance up. Nothing to do with the way the ground slopes. If the ground slopes down after you lift off, goodo, you are safer. But a positive gradient is a distance you climb reference MSL (ie a rising altimeter meaning a pressure drop due altitude rising) versus the distance forward it takes to climb.

However, the first segment doesn't apply whilst you are on the ground so that comment is irrelevant.

In other words, the altimeter is increasing.

Isn't that the same as "the altimeter is increasing"?

Most of the time, you won't be over the runway during the first segment, but over the ground beyond the end of the runway (stopway/clearway, splays), so the question is almost entirely moot.

Hi John.
We have an aircraft which broadly resembles the Argosy in your description, and is normally first segment limited - and retraction is normally only complete at 300ft.

But if your engine fails at V1 on the runway and your "positive climb" is 0.1% and the runway is sloping up at 0.2%, you cannot get airborne.
Our aircraft is similar in general description to John's Argosy and is first segment limited, and in a normal takeoff gear retraction is only complete by 300ft.

retraction is normally only complete at 300ft

Now that's an interesting one. Perhaps you can PM me with details for an off line discussion ?

One needs to keep in mind that first segment (and all the segments) is relevant OEI only. As a general rule, we ensure that the AEO profile is above the OEI profile and all's well.

"positive climb" is 0.1% and the runway is sloping up at 0.2%, you cannot get airborne.

Probably not quite that problematic but it's good to see your operator is mindful of the consideration

PM'ed John

Not quite right. The first segment starts at a point known as "reference zero", which is at 35' above the runway and where the takeoff is deemed to have ended - it does not start on the runway.

If the TODA for the runway was greater than or equal to the TODR from your performance calculations, then you are "guaranteed" to get airborne, having lost an engine at V1, and be at 35' before the start of the (rising) obstacle surface at the end of the clearway. This is the point where the first segment starts.

But from there on, there is no guarantee you won't collide with obstacles. It all depends on your first and second segment performance and the slope of the obstacle surfaces over which you are flying.

Sounds like this is just a reproduction of FAR25 (or equivalent) certification requirements, in which case the first segment commences at the END of the takeoff procedure (i.e. 35 feet dry/15 feet wet above the departure end of the runway), and is essentially just a requirement that the aircraft doesn't sink while the gear retracts. Runway slope is a factor for takeoff, but not climb. Obstacles in the departure splay must of course be taken into account, but this has nothing to do with runway slope.

If the TODA for the runway was greater than or equal to the TODR from your performance calculations, then you are "guaranteed" to get airborne,

Depending on the rules applicable to the certification, TODR will have up to half or two thirds the airborne distance to screen beyond the end TORR. This is to give you some comfort that the aircraft should be airborne by the end of TORR. Guarantee ? pretty likely but prefer not to figure in guarantees ...

before the start of the (rising) obstacle surface at the end of the clearway.

No warranty that the clearway won't have some slope associated with it ...

It all depends on your first and second segment performance and the slope of the obstacle surfaces over which you are flying.

Which is why it is prudent to consider first segment gradient and ground slope in the relevant part of the takeoff.

Sounds like this is just a reproduction of FAR25

Pretty much the case. 20.7.1B originally was the operational recasting of the Australian certification requirements. ANO 100.5 reflected the BCAR while 100.6 FAR 25 - both now well and truly binned post Ron Yeats' involvement.

and is essentially just a requirement that the aircraft doesn't sink while the gear retracts.

That's the certification line in the sand.

Runway slope is a factor for takeoff, but not climb. Obstacles in the departure splay must of course be taken into account, but this has nothing to do with runway slope.

As a general consideration, that is a reasonable position. However, for those aircraft which have an abysmal first segment performance, takeoff over rising ground (doesn't matter whether it is the runway or beyond the runway head) warrants some risk management going a little bit beyond just applying the AFM requirements.

Thanks John for the clarifications and summary. I absolutely agree with your point about risk management, although I'd emphasis (as you've alluded to) that minimum regulatory requirements should not form the sole basis for operational risk management - particularly the more prescriptive style rules of which you still seem to have a few in Australia (although, pleasingly, CASA is slowly learning about performance-based oversight/regulation from their good friends across the ditch :}). There will likely be some other rule requirement that a multi-engine aircraft on revenue passenger operations must 'clear all obstacles in the takeoff flightpath' after suffering a failure of the most critical engine at the most critical point, and that's probably the catch for this particular issue I'd think.

I'll just quote it as I remember it without refreshing my memory. The first segment for a two engined aircraft must be positive (there is no minimum climb gradient) from airborne until the gear is retracted. The certification requirement for subject aircraft types the gear must be fully retracted within 12 seconds.

Just a minor observation .. The first segment for a twin-engined aircraft must be positive (there is no minimum climb gradient) from airborne until the gear is retracted

True, regarding the gradient. However, the first segment, proper (if it exists at all), commences from screen and continues until the gear retraction is completed. Again, minimum WAT gradient is zero.

I always enjoy the discussions that pop up every now & then about climb grad etc; makes for interesting reading, glad we don't have crashes every day though!:-)

All well and good stating that first segment starts at 35ft - but you must get there first.
Your TODR gets you from on the runway to that 35ft point, and the transition from "on the runway" to "35ft" must be at your first segment performance.
So surely that means you must be able to climb at first segment requirement with the runway slope factored in to achieve TODR? Do you have to refer to the First Segment with runway slope factored in to calculate an initial TOW, before only then using that weight to calculate TODR,ASDR?