Driftdown confusion

[A1TAPE]

I made this in my notes after reading about driftdown in the asa book.
Methods for complying with driftdown regs


****Applies to A/C certified AFTER 8/29/1959****

Dispatch A/C with a weight that will

• Allow it to climb
  
  
• Allow it to clear all obs within 5 SM either side by 1000 feet vertically
  
  
• Allow it to climb when it reaches its diversion airport at 1,500 feet

Used when A/C IMMEDIATLY descends from planned cruise alt to driftdown service ceiling





****Applies to A/C certified AFTER 9/30/1958

Dispatch A/C with a weight that will

• NOT PROHIBIT a net flight path that:
  
  • Clears all obs within 5 SM either side BY 2,000 FEET
    
    
  • Allow it to climb when it reaches its diversion airport at 1,500 feet

Used when A/C GRADUALLY descends from planned cruise alt to driftdown service ceiling using a curved path WITH THE FOLLOWING ASSUMPTIONS MADE:

• Engine failure occurs at most critical route point
  
  
• A/C will pass a critical obs and will be equal distance or closer to a NAVAID than the CO (critical obs)
  
  
• MOST ADVERSE winds are used for planning
  
  
• Fuel dumping will occur if applicable
  
  
• Legal enroute alternate is specified
  
  
• Fuel+oil consumed can be considered when determining flight path
  
  • All this means is that you get credit for the plane losing weight while it descends

Driftdown requirments for 2 engine out enroute

***Applicable to aircraft with MORE than 2 engines AND certified AFTER 8/29/1959***

To meet this requirement, the A/C must NEVER:

• Be > 90 minutes from a compliant 121.197 airport
  
  • A .197 airport has a runway long enough that the aircraft WILL STOP WITHIN (60% for turboJETS and 70% for turboPROPS) of the runway length when it arrives
• Exceed a weight that prevents it from maintaining a net flight path to clear the highest obs 5 SM either side BY 2,000 FEET

In addition the following assumptions are made that the aircraft WILL:

• LOOSE BOTH ENGINES at the MOST CRITICAL flight point
  
  
• Have a positive climb at 1,500 feet above the alternate airport
  
  
• Have enough fuel to fly at this altitude/location AT LEAST 15 minutes
  
  
• Conduct fuel dumping if applicable and consumptions can be considered

The part that im confused about is when would you use the 2nd 1 engine out method? Also why are there 2 different methods for different years? Since almost all of todays aircraft for 121 ops were certificated well after the dates in the regs which one do you use? Would it say in Op Specs which one to use or does it depend on the route? In the ASA book it said that

Thus for simplicity, method one is easiest to calculate and use. Unfortunately, in some parts of the world, this simpler method might prohibit certain routes, so the more complex method might have to be utilized to take advantage of the more gradual driftdown that aircraft actually make in such an emergency.

Ison, David C.. Aircraft Dispatcher Oral Exam Guide (Kindle): Prepare for the FAA Oral and Practical Exam to Earn Your Aircraft Dispatcher Certificate (Oral Exam Guide Series) (Kindle Locations 927-929). Aviation Supplies and Academics, Inc.. Kindle Edition.

Anyone have a way to make it simple? I know the basics of what driftdown does but the regs are hard to make sense of.

 

[Delta Echo]

The first two are what we refer to as method 1 and 2 both are designed to prove continued flight after the loss of one engine. Method 1 is intended for use when the aircraft is in cruise and loses an engine. It is simply a way of assuring terrain clearance after descending to an altitude that the remaining engine can support the aircraft's weight.

Method 2 is generally used when terrain clearance can't be assured and a drift-down alternate has to be applied somewhere before the critical terrain point.

 

[pljenkins]

Basically it works like this.

For the first part, pre 1959, you must plan a route that allows for terrain clearance of 1000 feet along the entire route of flight with an engine out. As you noted, this is largely not used by us as pretty much everything we push is post 1959.

For later aircraft, you have two options:

First, known as "method 1", is effectively the same as the pre-1959 regs. You must be able to clear all obstacles by 1000 feet for the entire flight path with an engine out. Easy peasy.

Second, known as "method 2" comes into play if your weight and/or flight plan cannot comply with "method 1". For method 2, you must have a flight plan that allows for 1000 foot obstacle clearance at any point on the route. To accommodate that, you may need to assign alternates for segments where that clearance is not possible that will ensure that at any given moment there is a plan that guarantees such terrain clearance.

 

[bbmikej]

It all depends on the route that the aircraft is taking and which aircraft it is.

Most of the time with the aircraft I dispatch and the region we are in being the eastern US method 1 is used. There is a certain route though (i cannot remember which exactly it is) that goes near the highest point in the Appalachian Mountains that technically requires method 2. Method 2 is mostly seen in the mountains where the peaks are higher than the 1 engine cruise altitude of the aircraft. For example a route like DFW to GJT will have to cross a line of the Rocky Mountains that an aircraft cannot maintain 1000ft clearance with 1 engine along the route. For this you would use method 2 and list an alternate (or multiple alternates) along the route that the aircraft can get to from that point with 1 engine out and maintain 2000ft clearance. Just remember that that airport has to meet alternate requirements to be listed unless is is the destination airport.

The flight planning software will automatically decide which method is to be used and will list which airports are being used for method 2, just make sure that it isn't using an illegal alternate airport.

 

[Deleted member 27505]

I was going to say "just fly an Apache and never take off".
Then I put 2 and 2 together. 1959 was the year the PA-23 got engines big enough to keep it in the air after one quit.

 

[Flying Saluki]

I made this in my notes after reading about driftdown in the asa book.
Methods for complying with driftdown regs


****Applies to A/C certified AFTER 8/29/1959****

Dispatch A/C with a weight that will

• Allow it to climb
  
  
• Allow it to clear all obs within 5 SM either side by 1000 feet vertically
  
  
• Allow it to climb when it reaches its diversion airport at 1,500 feet

Used when A/C IMMEDIATLY descends from planned cruise alt to driftdown service ceiling





****Applies to A/C certified AFTER 9/30/1958

Dispatch A/C with a weight that will

• NOT PROHIBIT a net flight path that:
  • Clears all obs within 5 SM either side BY 2,000 FEET
    
    
  • Allow it to climb when it reaches its diversion airport at 1,500 feet

Used when A/C GRADUALLY descends from planned cruise alt to driftdown service ceiling using a curved path WITH THE FOLLOWING ASSUMPTIONS MADE:

• Engine failure occurs at most critical route point
  
  
• A/C will pass a critical obs and will be equal distance or closer to a NAVAID than the CO (critical obs)
  
  
• MOST ADVERSE winds are used for planning
  
  
• Fuel dumping will occur if applicable
  
  
• Legal enroute alternate is specified
  
  
• Fuel+oil consumed can be considered when determining flight path
  • All this means is that you get credit for the plane losing weight while it descends

Driftdown requirments for 2 engine out enroute

***Applicable to aircraft with MORE than 2 engines AND certified AFTER 8/29/1959***

To meet this requirement, the A/C must NEVER:

• Be > 90 minutes from a compliant 121.197 airport
  • A .197 airport has a runway long enough that the aircraft WILL STOP WITHIN (60% for turboJETS and 70% for turboPROPS) of the runway length when it arrives
• Exceed a weight that prevents it from maintaining a net flight path to clear the highest obs 5 SM either side BY 2,000 FEET

In addition the following assumptions are made that the aircraft WILL:

• LOOSE BOTH ENGINES at the MOST CRITICAL flight point
  
  
• Have a positive climb at 1,500 feet above the alternate airport
  
  
• Have enough fuel to fly at this altitude/location AT LEAST 15 minutes
  
  
• Conduct fuel dumping if applicable and consumptions can be considered

The part that im confused about is when would you use the 2nd 1 engine out method? Also why are there 2 different methods for different years? Since almost all of todays aircraft for 121 ops were certificated well after the dates in the regs which one do you use? Would it say in Op Specs which one to use or does it depend on the route? In the ASA book it said that



Anyone have a way to make it simple? I know the basics of what driftdown does but the regs are hard to make sense of.

Method 1 (FAR 121.191 (a)(1)) assumes an engine failure at V1, so you must take off at a weight that allows you to lose an engine at takeoff and maintain the specified terrain clearances. Method II (FAR 121.191 (a)(2)) assumes an engine failure at cruise altitude and the aircraft drifts down to an altitude where it can maintain the appropriate terrain clearances. I like to think of it as climb-up vs. drift down.

Generally speaking, Method II allows you to carry more weight, but also requires additional workload on the part of the dispatcher and crew. Method I is easier from a dispatcher workload point of view, but allows for less payload.

 

[A1TAPE]

Method 1 (FAR 121.191 (a)(1)) assumes an engine failure at V1, so you must take off at a weight that allows you to lose an engine at takeoff and maintain the specified terrain clearances. Method II (FAR 121.191 (a)(2)) assumes an engine failure at cruise altitude and the aircraft drifts down to an altitude where it can maintain the appropriate terrain clearances. I like to think of it as climb-up vs. drift down.

Generally speaking, Method II allows you to carry more weight, but also requires additional workload on the part of the dispatcher and crew. Method I is easier from a dispatcher workload point of view, but allows for less payload.

But for method 1 the book says to think of it

as if the aircraft falls immediately from its cruise altitude to its new service ceiling.

Ison, David C.. Aircraft Dispatcher Oral Exam Guide (Kindle): Prepare for the FAA Oral and Practical Exam to Earn Your Aircraft Dispatcher Certificate (Oral Exam Guide Series) (Kindle Location 921). Aviation Supplies and Academics, Inc.. Kindle Edition.

 

[who'swho]

The thing that made the 2 clear to me was when I was in recurrent and was told method 1 starts at V1 and method 2 starts at top of climb. Guess I had missed that part initially. It sounds super complicated the way the regs word it.

 

[Flying Saluki]

But for method 1 the book says to think of it

I haven't read the book, but it seems to me the operative word is "immediately". The airplane was at, say, 25000, and suddenly it's down to 15000. That's not driftdown. Driftdown assumes that the airplane comes down slowly, which takes into consideration the lateral movement of the aircraft as well as the vertical. That's my take on it, anyway.

 

[A1TAPE]

The easiest way to distinguish between these two methods is to think of the first as if the aircraft falls immediately from its cruise altitude to its new service ceiling. The second method assumes that the aircraft slowly drifts down following a curved path to its new service ceiling.

Ison, David C.. Aircraft Dispatcher Oral Exam Guide (Kindle): Prepare for the FAA Oral and Practical Exam to Earn Your Aircraft Dispatcher Certificate (Oral Exam Guide Series) (Kindle Locations 920-922). Aviation Supplies and Academics, Inc.. Kindle Edition.