(SIDs and DPs) 152 Feet per NM...

ldubsgmoney

Well-Known Member
I'm an instrument student learning about SIDs and DP's. My instructor says he'll give me bonus points if I can answer the following question:

what is the significance of 152 FPNM?

i took 152/6076 and got a 3% climb gradient. does this mean something special?

Anyway thanks for any advice.
 

SpiraMirabilis

Possible Subversive
I believe 152ft/nm is the standard minimum climb gradient on far 25.121 for for four engined aircraft.

"Take off, Landing gear retracted.....the steady gradient of climb may not be less than 2.4 percent for two engine airplanes, 2.7 percent for three engine airplanes, and 3.0 percent for four engine airplanes at V2..."
 

germb747

Well-Known Member
The way I understand it is when the field is TERPS, you draw a plane from 35ft over the departure end of the runway at a slope of 152 ft / nm, and if any obstacles penetrate that plane, then a required climb gradient will be published.

At Air Force bases, the plane starts at 0ft over the DER, and in the C-5 we're required to meet 200' / nm climb gradient on four engines and 152' / nm climb gradient on three engines. So with all engines operating, you get a 48' buffer per nautical mile clearance over obstacles.

Perhaps others more knowledgable than myself can pitch in; I'm too lazy to pull out the AIM and Instrument Flying Handbook right now but there's some good stuff in there too.
 

Rocketman99

Frozen Guppy Manipulator
That's pretty much it IIRC. TERPS criteria for clearance is that no obsacles penetrate the 152'/nm gradient in the departure. If something does then the obstacles are presented and higher than standard takeoff mins or climb gradients are issued...but I haven't looked at that stuff in a while so doublecheck.
 

Polar742

All the responsibility none of the authority
I'm an instrument student learning about SIDs and DP's. My instructor says he'll give me bonus points if I can answer the following question:

what is the significance of 152 FPNM?

i took 152/6076 and got a 3% climb gradient. does this mean something special?

Anyway thanks for any advice.
152/6076 = 2.5%

2.5% is equal to the required performance on second segment.

There's more technical crap (reference the above posts.), and the TERPS and Part 25 are involved, but that's the number for OEI on Takeoff. I'd write more, but grayson is a far better explainer than I can ever hope to be.

Tell Mitch he owes me a beer.
 

troopernflight

Well-Known Member
Or you can just compute your rate of climb required (gs/60)*fpnm. Typical climb airspeed for the DA-20 is 85 knots (give or take on actual gs), 85/60=1.42, and 1.42*152= 305fpm rate of climb required for obstacle clearance. Don't know what the significance is though.
 

ldubsgmoney

Well-Known Member
Hahaha I guess the cat's out of the bag on the whole Purdue thing. Thanks for all the advice, and I'll tell Mitch to open a tab at Harry's.
 

RynoB

That One Guy
I'll add to what has been discussed. The standard IFR climb gradient is 200'/nm, or 3.3%. They take the 152'/nm that has been discussed from the end of the runway, and make sure no obstacles protrude into that path. They then add 48'/nm for a buffer to get the 200'/nm. It makes sense if you think about it. If your aircraft could meet exactly 152'/nm under the conditions, you could potentially scrape the belly on that obstacle that is right at the limit. Hence the extra 48'/nm.

In the past, the clearance path started at 35' above the departure end threshold. New TERPS certification procedures dictate that it starts from ground level. So some departures will be based on the old criteria from 35', and others from the surface.

Refer to the chapter covering departures in the Instrument Procedures Handbook for more info. I don't have the book in front of me, so I can't quote the chapter or paragraph.
 

Rocketman99

Frozen Guppy Manipulator
Really, when did they change it for civil SIDs to start at 0? That was one of the pain in the arse things to remember in UPT. For the AF it was 0' but for civil SIDs (and Navy maybe?) it was 35'.
 

RynoB

That One Guy
I think it was changed in '07. That was when I first read about it. But it will take time to re-survey many of the airports, so most will use the 35' criteria for some time. I will go to the FAA Website to see if I can find the link. I will post it here.
 

RynoB

That One Guy
http://www.faa.gov/library/manuals/aviation/instrument_procedures_handbook/media/CH-02.pdf

Chapter 2 of the Instrument Procedures Handbook includes departure information. TERPS design criteria for departures is discussed on 2-12 through 2-17.

Below is part of the excerpt for the old and new TERPS criteria. The bold is mine for emphasis. There is a diagram for both criteria included in chapter that is linked above.

DESIGN CRITERIA
The design of a departure procedure is based on TERPS,
a living document that is updated frequently. Departure
design criterion assumes an initial climb of 200 feet per
nautical mile (NM) after crossing the departure end of
the runway (DER) at a height of at least 35 feet. [Figure
2-15] The aircraft climb path assumption provides a
minimum of 35 feet of additional obstacle clearance
above the required obstacle clearance (ROC), from the
DER outward, to absorb variations ranging from the
distance of the static source to the landing gear, to differences
in establishing the minimum 200 feet per NM
climb gradient, etc. The ROC is the planned separation
between the obstacle clearance surface (OCS) and the
required climb gradient of 200 feet per NM. The ROC
value is zero at the DER elevation and increases along
the departure route until the appropriate ROC value is
attained to allow en route flight to commence. It is
typically about 25 NM for 1,000 feet of ROC in nonmountainous
areas, and 46 NM for 2,000 feet of ROC
in mountainous areas.
Recent changes in TERPS criteria make the OCS lower
and more restrictive. [Figure 2-16 on page 2-14]
However, there are many departures today that were
evaluated under the old criteria [Figure 2-15] that
allowed some obstacle surfaces to be as high as 35 feet
at the DER. Since there is no way for the pilot to determine
whether the departure was evaluated using the
2-13
previous or current criteria and until all departures have
been evaluated using the current criteria, pilots need to
be very familiar with the departure environment and
associated obstacles especially if crossing the DER at
less than 35 feet.
 

germb747

Well-Known Member
Does this have anything to do with whether an airport meets diverse departure criteria?
If an airport has an approved instrument approach procedure, you can look in the front of the approach plates to see if the airport is covered by a "trouble T". If there's no obstacle departure procedure (ODP) then you should be okay doing a diverse departure (climb to 400 feet then turn on course with a minimum climb gradient of 200' per nautical mile).

If the airport doesn't have an approved instrument approach procedure, then I couldn't tell you how to depart safely under IFR because there won't be a trouble T published for those airports even if there are penetrating obstacles.
 
Top