A320 single-engine driftdown procedure

[sabenaboy]

In the picture below you can find a driftdown table for an A320. (FCOM 3.06.40 p3)
http://home.scarlet.be/~kd6qe2mr/driftdown.jpg
It's pretty straightforward to use.
Ex.: With an engine Failure at FL350 at a GW of 70 tons and applying the drriftdown procedure I will be able to level off at 20900 ft, reached after 69 mins and 376 NM. By then I will have used 2.300 kgs of fuel and flown initially at a green dot speed of 240 kts. (Flown with Autothrust off and remaining engine at MCT)
But what amazes me is the influence that anti-ice will have on these figures. (last two lines)
The level-off altitude decreases only slightly and the fuel consumption increases: that sounds logical to me so far. But when putting engine or wing anti-ice on, the distance and time to reach level-off INCREASES SIGNIFICANTLY and that amazes me a little bit.
Now I can try guessing why this occurs, but I was hoping somebody would know how this can be explained.
Regards,
Sabenaboy

[SMOC]

I'll have a guess that it's the +15% increase in fuel consumption which means the A/C will be getting lighter during the descent than it would be with AI off.

[oldebloke]

assuming the Autothrust is 'off'with the power lever in the MCT detent,I can only assume the 'higher' fuel burn would account for the 'higher' s/e ceiling,as above..:yuk:

[Old Smokey]

sabenaboy, what you have to bear in mind is that whilst the Level-Off Altitude may be a mere 300 feet lower, the last couple of hundred feet of any Drift-Down is at an extremely low Rate of Descent. This is quite normal

I've Programmed and produced Drift-Down charts for several aircraft types, and, typically, the first 10,000 to 15,000 feet of Drift-Down happens pretty fast (for 2 engined aircraft), but as the descent progresses, with decreasing Rate and Gradient of Descent, approaching zero at the Level-Off Altitude, Distance, Time, and Fuel increase for each foot of altitude lost. That last 300 feet of Drift-Down will probably be at a mean descent rate of about 30 feet per minute, decreasing to zero. Thus, although the Level-Off Altitude is lower, a lot of distance is covered for those 300 feet. If the 'Normal' table were to show you incremental Distance, Time, and Fuel between altitudes (it doesn't) you would see that the last part of the Anti Ice OFF descent also covers a considerable distance.

The increments for Total Anti Ice ON at Distance +14% : Time +13% : Fuel +15%, are nearly equal, thus, excepting a VERY small fuel penalty (+15% Vs +14%), it is more representative of a longitudinally extended Drift-Down for a slightly extended Vertical component. Very very typical of all aircraft.

Regards,

Old Smokey

[AeroTech]

Hi,

I am not familiar with this drift-down table. So this just a simple guess:
The MCT during drift-down with engine anti-ice ON is superior than the MCT with total anti-ice ON (IF total anti-ice means engine and wing anti-ice, I guess the other electrical anti-ice are always ON like Pitot..), and MAY BE that's why the distance and time to reach level-off INCREASES SIGNIFICANTLY (the same reasoning for the MCT with engine anti-ice OFF compared to the MCT with engine anti-ce ON). The raison is wing anti-ice represents an important load for engine compared to the engine-anti-ice.

If what I said is true, the fuel consumption with total anti-ice ON should be superior than the fuel consumption with engine anti-ice ON. But infortunately:ugh: it is the same on both cases (+ 15 % according to the table). Sorry Sabenaboy I cannot help you :{
Can someone explain me why the fuel consumption is the same (+ 15 %) on both cases: engine anti-ice ON & total anti-ice ON?

Thank you.
Best regards.

[Old Smokey]

Can someone explain me why the fuel consumption is the same (+ 15 %) on both cases: engine anti-ice ON & total anti-ice ON?

It's not the same, but nevertheless not much worse. The increment for fuel is +15% Versus +14% for the distance, i.e. 115% Vs 114% total, an A.N.M.P.K. decrease (penalty) of 0.88%

Sabenaboy, a diagram which might visually explain the situation follows within 24 hours.

Regards,

Old Smokey

[sabenaboy]

Thanks for the replies. Looking forward to it, old smokey.

[Old Smokey]

Sabenaboy,
Is a picture worth a thousand words?
http://static.flickr.com/41/108897790_391a13fcf7_o.jpg
Regards,
Old Smokey