FAR 25.121 vs. AIM

[leardvr]

FAR 25.121 says that for a single engine missed approach a 2 engine a/c must climb at 2.1%.

The AIM say 200'/nm or a 3.3%.

Where does the AIM get there number from?

If it's the TERPS please provide a link.

Can I assume that I should follow the AIM if it's more restrictive. Even though it's not regulatory.

Thanks

 

[Andy Neill]

TERPS says the obstacle plane has a 40:1 slope (2.5%) from the MAP. See page 26 of the manual (not the pdf document) from the link below.

http://www.faa.gov/about/office_org...s_guidance/orders/media/Order 8260.3_1-18.pdf

 

[leardvr]

Thanks for jogging my memory at 3am. What's the deal with 2.1 v 2.5 then. If I climb at 2.1 as per part 21 will I hit something?

 

[Doc Holiday]

The approach climb requirement of 2.1% is only a value required for certification of transport category airplanes. It is not interrelated with TERPS criteria. Another example is the second segment required climb gradient of 2.4% (two engine aircraft). The airplane must be able to meet that gradient to be certified under Part 25. To be assured obstacle clearance, climbing atleast 200 feet per NM (3.3%) would be needed for standard instrument operations.

What the aircraft needs for certification and what is required for instrument procedures are two different animals.

 

[leardvr]

What the aircraft needs for certification and what is required for instrument procedures are two different animals.

That's exactly what I think too. BUT every part 135 company I've ever flown for has said a 2.1 is required for a SE miss to the "safe altitude" both in the Ops Specs and the FOM.

So how did they get there POI to sign off on that?

And the first question remains, where did the AIM get 3.3?

 

[Doc Holiday]

I am curious about where in your Ops Specs this is specified. The FOM is written by the company and may contain that language. And unless you are using a performance vendor to calculate takeoff and landing weights, I'm not sure what "safe altitude" is referring to.

It looks like you are flying the Lear right now, and if I remember correctly the AFM states three criteria for calculating the max landing weight. The maximum structural landing weight, the maximum weight allowed for the runway length, and the maximum weight that will still allow approach and landing climb gradients (2.1% and 3.2%) to be attained. The most restrictive of these weights is controlling. That's the Part 25 requirements, so if you exceed one of these limits (which is usually the approach or landing climb), the airplane is being operated outside of its limitations.

So the 2.1% approach climb requirement is just to be sure you are operating the aircraft within it's limitations. To tie this in with TERPS, there are companies that do an airport analysis and create single engine procedures for specific aircraft that will deliver you to a safe altitude in case you bag an engine.

For the AIM question, the way I understand it is that the 40:1 plane (152' per nm) is the standard plane that no obstacles may penetrate. Climbing at 200' per NM as specified in the AIM gives you a buffer between the airplane and the obstacles that extend to the 40:1 plane.

Hope this helps. Also wanted to point out that this general topic has been discussed previously. I can't remember the title of the thread, but Lead Sled has a few detailed posts that are very informative.

 

[leardvr]

Seems reasonable enough. And is right in line with what I was thinking. Basicly operating the A/C within it's limitations, via Part 125, and via company limitations May plant me squarely into rising terrain on a missed. Very often going into KASE in a XL we plan the flight for the 2.1 missed. Here we go again, legal but not safe.

 

[MauleSkinner]

If you're VMC, you can avoid obstacles by turning...the lower "certification" gradients are generally acceptable.

If you're IMC, you can't "see and avoid" obstacles, so you should probably meet the AIM criteria (or higher, depending on published obstacle requirements).

Fly safe!

David