TDP and CDP
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TDP and CDP
I wonder if some of you more learned guys out there can throw some light on this subject. I know they mean almost the same thing but there is a difference. Is there any defination in the JAR Ops?
Take off Decision Point and Critical Decision point. Point at which, if an engine failed, you either decide to continue or land. It's just a 'decision' point during the take off (or landing), and AFAIAA they mean the same thing. TDP is just a new way of saying it. CDP was the old way of saying it. Same as LDP/CDP
Is that about right or am I talking b******s?
Is that about right or am I talking b******s?
The change came in CFR 14 Part 29 amendment 39.
Part 29 is the Certification standard for large rotorcraft (and CS29 the EASA equivelant), and it includes the requirements for Cat A performance. Eralier amendment referred to CDP and from 39 onwards TDP. From Joe line pilot's perspective, no difference!
If you want to read more, look up Part 29 and also AC29-2C (just google, with FAA thrown in to narrow it down)
Part 29 is the Certification standard for large rotorcraft (and CS29 the EASA equivelant), and it includes the requirements for Cat A performance. Eralier amendment referred to CDP and from 39 onwards TDP. From Joe line pilot's perspective, no difference!
If you want to read more, look up Part 29 and also AC29-2C (just google, with FAA thrown in to narrow it down)
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In my opinion, i believe that there is a slight difference between TDP and CDP. CDP is derived from the charts where you use either speed or time for the CDP.
TDP, eg if you are taking off from a long runway. You could be passed your CDP but because there is still sufficient runway remaining for you to land safely in the event of OEI. So, my TDP will be a point where i will deem not able to land the aircraft due to insufficient runway length corresponding to the speed and height at that particular point (where this is mainly DR) for me.
I stand to be corrected
TDP, eg if you are taking off from a long runway. You could be passed your CDP but because there is still sufficient runway remaining for you to land safely in the event of OEI. So, my TDP will be a point where i will deem not able to land the aircraft due to insufficient runway length corresponding to the speed and height at that particular point (where this is mainly DR) for me.
I stand to be corrected
Stand duly corrected! See post 3
The Veloceraptor of Lounge Lizards
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212man;
Cougar may be right. On the AS332 on take off from a runway, if I remember correctly we used to call TDP as the point the aircraft passed the single engine fly away speed and CDP was the point at which we couldn't reject the take off at all. One was speed based, and the other height and runway length remaining.
A Puma past VTOSS with 5000 feet of runway remaining could easily be flared and the TO rejected. I stand to be corrected as it is quite a while since I flew the Puma.
VH
Cougar may be right. On the AS332 on take off from a runway, if I remember correctly we used to call TDP as the point the aircraft passed the single engine fly away speed and CDP was the point at which we couldn't reject the take off at all. One was speed based, and the other height and runway length remaining.
A Puma past VTOSS with 5000 feet of runway remaining could easily be flared and the TO rejected. I stand to be corrected as it is quite a while since I flew the Puma.
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Totally agree with flungdung, if you reject after TDP and something goes wrong, i.e. you run out of runway, whatever, it's going to be your fault and you better have some very good reasons for your actions. A gentleman/woman in a black gown and funny wig is going to be asking you some very pertinent questions to try and make sure that everything that hits the fan lands (and sticks) to you. That's blame culture for you, it's the society that we live in, like it or not.
Could well culminate in a visit to 'Aviation House' to deliver licence to CAA shredder.
Could well culminate in a visit to 'Aviation House' to deliver licence to CAA shredder.
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Brom,
Don't forget that an RTO can be the result of many things going wrong, not just an engine failure. As I said in my original post CDP was treated as a runway length/height point on that particular operation. There always comes a point when there isn't enough tarmac left. The operations manual left a large amount to our discretion should we need to reject, but was very clear that runway length was the final factor. Slight drift, but the 748 pilot who had an uncontained engine burst at Stansted some years ago when already airborne rejected straight ahead onto the runway. He was pilloried by many who had no idea that it was an approved manouvre for the 748. Turned out he was very right due to a very weakened main spar which would not have survived a circuit.
I personally rejected a take off well after TDP due to a very sudden severe eyeball bouncing vibration. 4000' of runway left so no problem IIRC airspeed was about 100 kt when the desicion to re-land was made. MRB delamination was the problem. It could have been much worse had I taken it round the circuit.
VH
Don't forget that an RTO can be the result of many things going wrong, not just an engine failure. As I said in my original post CDP was treated as a runway length/height point on that particular operation. There always comes a point when there isn't enough tarmac left. The operations manual left a large amount to our discretion should we need to reject, but was very clear that runway length was the final factor. Slight drift, but the 748 pilot who had an uncontained engine burst at Stansted some years ago when already airborne rejected straight ahead onto the runway. He was pilloried by many who had no idea that it was an approved manouvre for the 748. Turned out he was very right due to a very weakened main spar which would not have survived a circuit.
I personally rejected a take off well after TDP due to a very sudden severe eyeball bouncing vibration. 4000' of runway left so no problem IIRC airspeed was about 100 kt when the desicion to re-land was made. MRB delamination was the problem. It could have been much worse had I taken it round the circuit.
VH
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Verticalhold,
You're quite right and I totally agree with you, that's why I said '...you better have some very good reasons for your actions'.
Good decision by you to reject in that instance,
this is where experience comes into it's own, the less experienced may have continued with disastrous results. Personally, I'd rather miss the runway completely and live to argue the case than have a MRB fall to pieces downwind, however, rejecting and running out of runway following an alternator failure or a drop in engine oil pressure may be a little more difficult to explain.
You're quite right and I totally agree with you, that's why I said '...you better have some very good reasons for your actions'.
Good decision by you to reject in that instance,
this is where experience comes into it's own, the less experienced may have continued with disastrous results. Personally, I'd rather miss the runway completely and live to argue the case than have a MRB fall to pieces downwind, however, rejecting and running out of runway following an alternator failure or a drop in engine oil pressure may be a little more difficult to explain.
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Brom;
One day the lawyers will all have been shot and we will be able to get on with our jobs without constantly looking over our shoulders.
Just think, only management to worry about
VH
One day the lawyers will all have been shot and we will be able to get on with our jobs without constantly looking over our shoulders.
Just think, only management to worry about
VH
Hi Flungdung
Since I have my pedantic head on, and if memory serves correctly! TDP is defined as the point at which the take off can either be rejected or continued. A nanosecond before or after is a different matter though.
Cheers
TeeS
Since I have my pedantic head on, and if memory serves correctly! TDP is defined as the point at which the take off can either be rejected or continued. A nanosecond before or after is a different matter though.
Cheers
TeeS
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When the rules changed, the acronym changed. They actually mean the same thing TDP=CDP
The old rules used LDP and so do the new rules.
This illustrates rule three of The Governing Rules:
1) Acronyms shall be used to obscure reality.
2) The best acronym shall have the worst technical justification.
3) When you need to look as if you have done something profound, rehash an old thing to make a new acronym
The old rules used LDP and so do the new rules.
This illustrates rule three of The Governing Rules:
1) Acronyms shall be used to obscure reality.
2) The best acronym shall have the worst technical justification.
3) When you need to look as if you have done something profound, rehash an old thing to make a new acronym
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CDP vs. TDP
More info on the subject:
New 14 CFR 29.55 Takeoff Decision Point: Category A
Notice 90-1 proposed to add this new section to redefine the takeoff critical decision point (CDP) previously contained in Sec. 29.53; it further proposed to remove the requirement to identify the CDP by height and airspeed, since height alone or other factors may be more appropriate. A commenter suggests that the section title and other references to ``critical decision point'' be changed to ``takeoff decision point (TDP).'' The commenter notes that TDP is compatible with the term ``landing decision point (LDP)'' already in other regulatory parts. The FAA agrees; accordingly, ``critical decision point'' is changed to ``takeoff decision point.''
Additionally, a commenter to Sec. 29.59 states that engine failure and the TDP do not occur at the same time because of necessary pilot- recognition time. The FAA agrees that a time interval for pilot recognition of the engine failure must be included when establishing the TDP. Calculating a pilot-recognition time interval when determining the TDP is a natural part of the TDP-determining process. Current industry practice already adequately considers this pilot-recognition time interval in determining the TDP. Therefore, to explicitly state this requirement in the regulations imposes no additional economic burden on manufacturers. Also, to harmonize Title 14 and the Joint Aviation Requirements (JAR's), the certification requirements for the Joint Aviation Authorities (JAA) of Europe, an explicit adoption of the pilot-recognition time interval is necessary. Therefore, since a pilot- recognition time interval is currently being used by manufacturers, and the FAA and the manufacturers are interested in harmonizing Title 14 and the JAR's, a new paragraph (c) has been added to Sec. 29.55 to require that a pilot-recognition time interval be included in the TDP determination.
New 14 CFR 29.55 Takeoff Decision Point: Category A
Notice 90-1 proposed to add this new section to redefine the takeoff critical decision point (CDP) previously contained in Sec. 29.53; it further proposed to remove the requirement to identify the CDP by height and airspeed, since height alone or other factors may be more appropriate. A commenter suggests that the section title and other references to ``critical decision point'' be changed to ``takeoff decision point (TDP).'' The commenter notes that TDP is compatible with the term ``landing decision point (LDP)'' already in other regulatory parts. The FAA agrees; accordingly, ``critical decision point'' is changed to ``takeoff decision point.''
Additionally, a commenter to Sec. 29.59 states that engine failure and the TDP do not occur at the same time because of necessary pilot- recognition time. The FAA agrees that a time interval for pilot recognition of the engine failure must be included when establishing the TDP. Calculating a pilot-recognition time interval when determining the TDP is a natural part of the TDP-determining process. Current industry practice already adequately considers this pilot-recognition time interval in determining the TDP. Therefore, to explicitly state this requirement in the regulations imposes no additional economic burden on manufacturers. Also, to harmonize Title 14 and the Joint Aviation Requirements (JAR's), the certification requirements for the Joint Aviation Authorities (JAA) of Europe, an explicit adoption of the pilot-recognition time interval is necessary. Therefore, since a pilot- recognition time interval is currently being used by manufacturers, and the FAA and the manufacturers are interested in harmonizing Title 14 and the JAR's, a new paragraph (c) has been added to Sec. 29.55 to require that a pilot-recognition time interval be included in the TDP determination.
Last edited by rotordude; 9th Nov 2007 at 15:10. Reason: More copy and paste
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For Cat "A" Elevated Heliport takeoffs, the S76C+ is lifted sharply and vertically out of the ground cushion to 50' Rad Alt; a tone then marks TDP and you immediately rotate smartly nose down to about 22 degrees. You are committed at the instant of cyclic input.
Yes Rotordude, a pilot recognition time interval is a necessary part of the process. But for this type of take-off you've got to go for it and you've got to nail the numbers; if you hesitate it will be messy.
No, the passengers don't like it much...
Yes Rotordude, a pilot recognition time interval is a necessary part of the process. But for this type of take-off you've got to go for it and you've got to nail the numbers; if you hesitate it will be messy.
No, the passengers don't like it much...
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I was told during my AS 332L2 conversion at Helisim that TDP is a fixed point which is predetermined based on ambient conditions so we have a fixed airspeed and ht above which we are to continue. For CDP, a good example would be take off from the rig where the CDP is the point of rotation. This point is not fixed by the numbers but more by pilot's feel ..and maybe not the most accurate point as it can vary slightly between pilots. I wonder if most of other pilots agree to this.
"Fixed Points"....strongly disagree to that. Example...45 knots of wind across the deck...empty aircraft in the North Sea in February compared to same aircraft in the Arabian Gulf in the Summer while at max weight and no wind to the same deck.
We are talking apples and oranges in that comparison.
Does that explain the wide difference of approaches being used around the world?
It always comes back to a variable decision point made by the pilot when making a decision as to being able to land vice go-around as affected by the circumstances.
Helicopter flying cannot be done by the "numbers" alone....otherwise there would be no need for pilots as autopilots could do the same job without all the attendant whinging and moaning.
We are talking apples and oranges in that comparison.
Does that explain the wide difference of approaches being used around the world?
It always comes back to a variable decision point made by the pilot when making a decision as to being able to land vice go-around as affected by the circumstances.
Helicopter flying cannot be done by the "numbers" alone....otherwise there would be no need for pilots as autopilots could do the same job without all the attendant whinging and moaning.
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SAS,
That's because what you're referring to regarding offshore decks, is mostly related to performance Class 2, though even for that RFM graphs give credit for wind and temperature. Very few helicopters are capable of operating Class 1 from an offshore deck with more than a few passengers. Many pilots will call committed at the point of rotation from an offshore deck, but that doesn't mean they can fly away, it just means that they're committed to continuing the take off in an attempt to gain sufficient airspeed to at least fly level, or even better start to climb before they enter the water. Under most jurisdictions, even Class 2 operations require that a twin engine aircraft must be capable of Class 1 performance at some point, often 1000 feet above the take off elevation.
TDP and CDP, as has been stated are basically the same thing. The DP is the essence; it's a decision point used for Class 1 operations. It's the only point at which in a multi-engine helicopter, the pilot may either decide to reject the take off, or continue it in the event of the failure of a critical power unit (after all the helicopter may have more than 2 engines and it may be able to climb on one or more engines after any one fails). If an engine fails before TDP/CDP the take off must be rejected and after it must be continued. In many companies this will be further modified inasmuch as there may be considerably more runway length available than that necessary for the relatively short accel/stop distance required by many helicopters (the distance required to accelerate to TDP speed and then safely recect and come to a stop on the prepared surface). If the Vtoss is low (CDP/TDP speed is usually related to Vtoss and a lower TDP speed means less space is required to safely reject the take off), the accel/stop distance will be correspondingly shorter. When there is a very long runway ahead the pilot may still be able to safely land back on even after TDP, though my experience with pilots under training shows that many less experienced pilots may find this difficult to achieve because we usually don't train much for this and it's a difficult thing to judge. You'd be surprised at the number of pilots who would have overrun the runway had I not restored an engine when under training. At least then pilots have an experience to relate to should it happen again for real. The TDP/CDP is really only critical when operating from a runway which is equal to the accel/stop distance for the weight of the aircraft in nil wind for a given temperature. This may well be the case when operating from small heliports, not fixed wing airfields.
Pauses for breath, has another cup oif strong coffee and goes off for a little sleep............
That's because what you're referring to regarding offshore decks, is mostly related to performance Class 2, though even for that RFM graphs give credit for wind and temperature. Very few helicopters are capable of operating Class 1 from an offshore deck with more than a few passengers. Many pilots will call committed at the point of rotation from an offshore deck, but that doesn't mean they can fly away, it just means that they're committed to continuing the take off in an attempt to gain sufficient airspeed to at least fly level, or even better start to climb before they enter the water. Under most jurisdictions, even Class 2 operations require that a twin engine aircraft must be capable of Class 1 performance at some point, often 1000 feet above the take off elevation.
TDP and CDP, as has been stated are basically the same thing. The DP is the essence; it's a decision point used for Class 1 operations. It's the only point at which in a multi-engine helicopter, the pilot may either decide to reject the take off, or continue it in the event of the failure of a critical power unit (after all the helicopter may have more than 2 engines and it may be able to climb on one or more engines after any one fails). If an engine fails before TDP/CDP the take off must be rejected and after it must be continued. In many companies this will be further modified inasmuch as there may be considerably more runway length available than that necessary for the relatively short accel/stop distance required by many helicopters (the distance required to accelerate to TDP speed and then safely recect and come to a stop on the prepared surface). If the Vtoss is low (CDP/TDP speed is usually related to Vtoss and a lower TDP speed means less space is required to safely reject the take off), the accel/stop distance will be correspondingly shorter. When there is a very long runway ahead the pilot may still be able to safely land back on even after TDP, though my experience with pilots under training shows that many less experienced pilots may find this difficult to achieve because we usually don't train much for this and it's a difficult thing to judge. You'd be surprised at the number of pilots who would have overrun the runway had I not restored an engine when under training. At least then pilots have an experience to relate to should it happen again for real. The TDP/CDP is really only critical when operating from a runway which is equal to the accel/stop distance for the weight of the aircraft in nil wind for a given temperature. This may well be the case when operating from small heliports, not fixed wing airfields.
Pauses for breath, has another cup oif strong coffee and goes off for a little sleep............
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TLAs and acronyms...
Nick as always is spot on with regards to acronyms.
To take the whole acronym thing to another level, the Oz military (and presumably others) actually called them TLAs (Three Letter Abbreviations).
I always thought having a TLA for a TLA was kind of funny (but then I am easily amused).
Papa68
To take the whole acronym thing to another level, the Oz military (and presumably others) actually called them TLAs (Three Letter Abbreviations).
I always thought having a TLA for a TLA was kind of funny (but then I am easily amused).
Papa68