Just a quick note to add. DDTOL (drift down take off limit) is sometimes used instead of ZNTOL (zero net take off limit) to allow a higher break release weight for reasons of enroute icing conditions reported/forecast.
example: ZNTOL is 74,000# due to enroute icing conditions (use of bleed air to heat wings = loss of engine performance/altitude capabilities on one engine for terrain clearance). Dispatch runs the numbers for a DDTOL to increase the weight allowable for T.O. (more weight=more passengers=more money). To allow this, they give you DD alternates. With this they give you "decision points" of X,Y,and Z. If you lose an engine before "X" you go to the appropriate alternate A. If you lose an engine between "X" and "Y" you go to alternate B. As you guessed it, an engine failure between "Y" and "Z" you go to atlernate C (usually your destination). This whole dispatch procedure allows you to comply with enroute terrain clearances covered nicely in the previous posts.
The drift down alternates, of course, have to comply with alternate WX minimums (forecasts) required for all other destination alternates.
One last note. If there is forecast icing conditions that require a limited ZNTOL, but you can get reliable pilot reports (read: from company aircraft that have flown on that route during the forecast conditions) that contradict/nullify the forecasts (they must be reported to dispatch by the crew), you can get a ZNTOL weight approval for the reported conditions (no DDTOL required to T.O at the higher weight).
All of this typing has made me very sleepy......must...get....rest...
example: ZNTOL is 74,000# due to enroute icing conditions (use of bleed air to heat wings = loss of engine performance/altitude capabilities on one engine for terrain clearance). Dispatch runs the numbers for a DDTOL to increase the weight allowable for T.O. (more weight=more passengers=more money). To allow this, they give you DD alternates. With this they give you "decision points" of X,Y,and Z. If you lose an engine before "X" you go to the appropriate alternate A. If you lose an engine between "X" and "Y" you go to alternate B. As you guessed it, an engine failure between "Y" and "Z" you go to atlernate C (usually your destination). This whole dispatch procedure allows you to comply with enroute terrain clearances covered nicely in the previous posts.
The drift down alternates, of course, have to comply with alternate WX minimums (forecasts) required for all other destination alternates.
One last note. If there is forecast icing conditions that require a limited ZNTOL, but you can get reliable pilot reports (read: from company aircraft that have flown on that route during the forecast conditions) that contradict/nullify the forecasts (they must be reported to dispatch by the crew), you can get a ZNTOL weight approval for the reported conditions (no DDTOL required to T.O at the higher weight).
All of this typing has made me very sleepy......must...get....rest...
