Compacted snow vs Level Dry Snow performance numbers

[medflyer]

Flying a 737-800 the other day we sent an ACARS performance request for takeoff using Level 1 Dry Snow. The thrust required came back as TO with Flaps 10. After deicing we received an updated RSC that now said the runway was now Compacted snow, a more restrictive condition as listed in our FOM. We re-sent the numbers and received back TO-2, Flaps 1. As it didn't seem right to depart with less thrust and Flaps than we had before with what was technically a worse runway condition I reverted back to what we had before.

Any idea why Conpacted Snow would require less thrust and flaps than the Dry Snow? Is it due to better acceleration on the hard picked surface vs the drag of the dry snow? Incidentally we had a higher MRTW with the CS and the limit changed from obstacle to Climb. This leads me to think the acceleration somehow benefits third stage climb, but I am missing a piece here. Can someone explain more clearly?

Thanks!

[agg_karan]

FOLLOWING IS THE EXTRACT FROM 'DRAFT, ICAO "Runway Surface Condition Assessment, Measurement and Reporting"

When the runway is covered by a loose contaminant (e.g., standing water, slush, non compacted snow), there are additional drag forces resulting from the displacement or compression of the contaminant by the wheel. The driving factors of these displacement drag forces are: aircraft speed and weight, tire size and deflection characteristics, and contaminant depth and density. Their magnitude can significantly impair the acceleration capability of the aircraft during take-off.


A second effect of these displaceable contaminants (slush, wet snow, and standing water) is the impingement drag, whereby the plume of sprayed contaminant creates a retardation force when impacting the aircraft structure. The combination of the displacement retardation force and impingement retardation force can be as high as 8 to 12 per cent of the aircraft weight for a typical small/mid size passenger aircraft. This force can be large enough that in the event of an engine failure the aircraft may not be able to continue accelerating.

Loose contaminants:
• acceleration capability reduced by displacement and impingement drag (slush, wet snow, and standing water) or the force required to compress the contaminant (dry snow); and
• deceleration capability reduced by lower friction, aquaplaning at high speeds, partially compensated by displacement and impingement drag

As a result,
• take off distance is longer (worse when contaminant is deeper);
• accelerate-stop distance is longer (less so when contaminant is deeper because of higher displacement and impingement drag);

Compact snow:
• the acceleration and continued takeoff is not affected;
• the accelerate-stop distance is increased by 30-60 per cent, reduced to 20-30 per cent with the use of thrust reversers (one-engine-inoperative)

The DOC also mentions about Wet ice, and other things very nicely. Can google and download it.
As to your question - Any idea why Conpacted Snow would require less thrust and flaps than the Dry Snow, I hope this helps.

Also you mentioned about DEICE and dispatch suggesting a derate later, our policy is to have no denates after DEICE/ANTIICE

Lastly you mentioned "This leads me to think the acceleration somehow benefits third stage climb, but I am missing a piece"
I am not too sure if I would relate the acceleration to third stage, if anyone has any explanation, more than welcome.
:)

[Skyjob]

DEICE and dispatch suggesting a derate later, our policy is to have no denates after DEICE/ANTIICE
There is no technical reason why one would not be able to use derated or even assumed thrust when having been deiced.

There are a variety of reasons for deicing requirements, but it is the runway conditions rather than the deicing requirement which allow/prohibit derated and ATM usage.

The difference of the OP's question/answer is the difference between Slippery/Contaminated runway characteristics, basically as summed up by previous poster.

Compacted snow can be seen as a slippery surface, whereas dry snow seen as a contamination form of the runway. In both cases performance should be assessed individually based on actual runway conditions and never be less then WET capability.

[lurkio]

IIRC from Boeing days if you used TO-2 you would achieve a lower VMCG and therefore could lift more weight. If your weight has not changed and using the same runway a lower flap setting could apply. This update from the mists of time so feel free to flame if wrong.

[PEI_3721]

As per agg_; in addition see the article in Safety First. (http://www.scribd.com/doc/252747808/Safety-First-19-Airbus-Flight-Safety-magazine-Jan-2015).
Other references; CS 25 AMC 25.1591, but I don’t recall an equivalent FAA version of this, also AC 91-71

Derate or lesser performance options should not be considered on a contaminated runway; the primary aim is for safety, not going with more load, etc, but stopping in the event of an RTO.

[agg_karan]

#skyjob
I totally agree with your point. In our manual it just clearly says so we follow it. :)

[medflyer]

Excellent info, thanks guys