TDP and CDP

[212man]

TLAs are a misnomer: acronyms should be pronouncable as a word e.g RADAR, LASER, HUD...

TDP and CPD (and TLA) are abbreviations.

[JimL]

Gnow,

The answer to your question was pointed to by 212man and precisely answered by 'rotordude' - simplistically, the change from CDP to TDP in amendment 29-39 permitted the decision point to be defined by a single parameter - e.g. height - instead of by speed and height. This in recognition that height might be the only parameter that can be defined within certain procedures (vertical, sideways or backup).

It was also the amendment that introduced 'pilot recognition time' - thus separating the point at which the engine is assumed to have failed from the point at which the decision is made; thus continued take-off is assured from TDP even with an engine failure up to one second before TDP (with some manufacturers, the recognition time is reduced to 0.5 second because of the efficacy of the engine failure warning system).

Early helicopter procedures followed fixed wing in that they encompassed an accelerate/stop distance and even used V1 as the point at which a Go/No Go decision had to be made. These procedures provided a speed and height for the V1 that kept a level accelerating helicopter out of the HV diagram.

It was only with the introduction of more powerful engines that the flexibility of the helicopter to do other than a fixed wing type take-off started to be explored. The S61 with its 'ground cushion' and 'oblique' take off procedures showed that the rejected take-off distance could be minimised - by introducing a vertical section - while preserving a reasonable take-off mass (an optimisation of the 'oblique' is still in use at Penzance today). Sikorsky also introduced a back-up procedure for elevated heliports but at a greatly reduced mass; Bell certificated a sideways (three dimensional) procedure for helidecks with the B212/412 that has been further developed for the B427 (and presumably the B429) - this is a good procedure because it preserves an extensive Field Of View (FOV) by utilising a sight picture out of the side window (this procedure is still one of the best because the improved FOV permits a take-off and approach angle of almost 90 degrees whilst preserving a full picture of the touch town and aiming point)

Eurocopter Deutschland produced a back-up procedure for the Bo105 which has been further refined for the other models in the EC range. Initially this profile was established with a fixed TDP but was subsequently improved by permitting a variable TDP (this variable TDP permits the helicopter to tolerate obstacles in the take-off distance by avoiding them vertically; as the TDP is raised, the 'mindip' is raised accordingly - i.e. if the fixed TDP of 50ft gives a 'mindip' of 15ft above the take-off site a TDP of 100ft will provide a 'mindip' 65ft above the take-off site). I'm sure all will see the potential of a raised TDP (if it is permitted in the procedure).

No need to linger on the PC2 offshore procedures as that has been well described by 'soggyboxers'; 'helicomparitor' has also explained PC2e in the S92 thread.

Operating PC1 using a Category A procedure from a (long) fixed wing runway provides no safety benefit. Equivalent safety can be shown by using a PC2 procedure - always providing that obstacle clearance is shown in the 'take-off flight path'. Performance in the 'take-off flight path' is identical for PC1 and PC2 (at that point the helicopter is operating in PC1 regardless of the take off procedure).

As a matter of interest, when the ICAO Heliops Panel started to work with the Performance Classes, they were defined as PC1a, PC1b, PC2 and PC3; PC1a being a procedure where a helicopter could remain in the hover on one engine. This (exceptional) performance was also anticipated by the FAA when they were amending FAR 29 back in the 1980s - this was (probably) the reason that ICAO and FARs were directed at 'a PC1 helicopter' and not at 'operations in PC1'. The complex PC1 designation was dropped before the introduction of the first comprehensive Annex 6 Part III and we have continued to develop procedures for PC2 in accordance with Risk Assessment.

Jim

[Chopper Jog]

CAT A Operations

AS332L2 data:

1. TDP(Take off Decision point) Definition - Abort the take off if an engine fails before the TDP and continue the take off if the engine fails after the TDP.

2. TDP is defined by a speed and height and is indicated by V1 (speed) and h1 (height). For example V1=20kts and h1=20ft, V1=30kts and h1=30ft, etc.

3. TDP is calulated as follow: VTOSS (IAS)-10kts=V1 (IAS)

Note: Minimum VTOSS=35kts TAS

[Max_Chat]

Munk. TDP is not at the point when translational lift takes place, it is a point during take-off when the ability to fly away on a single engine is assured. This could be height and/or IAS related.

Just because you happen to be at or beyond TDP when an engine fails does not necessarily mean that you are obliged to "fly away" with a given Continued Take-off Distance available.

IMHO

[Perro Rojo]

From a Line Pilot's point of view this is how I see it.

TDP(CDP) are OEI decision points only, as calculated for the current conditions. They are not a commitment to continue a Take-off given some other problem. That would be a pilot judgement call given the prevaling terrain conditions. Even the OEI decision could be affected by the terrain at the time of occurrance. If one is operating from 10,000 feet of runway an RTO after TDP would be prudent if it can be performed safely.

With the 332 in our operation we brief the Class of performance we are capable of, the perfomance Class profile we are going to fly (Pilot Flying discretion) and if Class 1 from a runway or Class 2 from a helipad or deck this includes a "Committed" to fly point by the Pilot Flying in additon to the TDP.

[Munk]

Max Chat
Yup, your definition is correct. Ergo it is not mandatory to continue the t/o with oei when TDP is reached, providing a reject can be safely completed - notwithstanding soggyboxers’ comments re training and other comments about lawyers. I can imagine a lawyer dryly querying, “was a fly away with an engine fire really necessary with 10,000 feet of runway remaining………”
I merely point out that the point at which TDP is reached coincides with the onset of translational lift. Other decision points, eg: rig take-off’s are committal points, not oei fly away points. A vertical take-off, or a helideck take-off use nose down fly away techniques with oei during which, of course, t lift is reached.

[Brom]

Monk,
In your post you say

I merely point out that the point at which TDP is reached coincides with the onset of translational lift.

That may be correct for some Helicopters/Take off profiles, however, it isn't true for all. As an example the Puma L2 (when operated to Cat A) can have a TDP which varies dependant on a number of different factors - take off space available, aircraft weight, temperature etc. I'm not going to go into detail on the calculation, but the TDP can be between 25 and 70 Knots IAS, well in excess of the onset of translational lift.
Brom.

[212man]

I merely point out that the point at which TDP is reached coincides with the onset of translational lift

Nonsense! ETL is typically around 17 kts and most IAS based TDPs range from 25-70 KIAS (few, if any civil aircraft can register accurate IAS values below 25 kts anyway). Not to mention that TDP can be purely height based with zero airspeed!

[Shawn Coyle]

Munk:
There is absolutely no connection between ETL and TDP.

[soggyboxers]

Munk,
TDP has nothing to do with translational lift. In a normal forward twin-engine take off in older helicopters such as 412/S76/365 (to mention but a few) it's the point at which you have reached, or can accelerate to your Vtoss. This is the airspeed for a given weight and temperature at which the helicopter has reached a point on the Power required/TAS curve where it has enough excess of power available over power required to be capable of climbing on one engine at 100 feet per minute (with landing gear down, on wheeled helicopters) using 2 minute power (on aircraft having a 2 minute power rating). Then at a point, normally 200 feet, the helicopter can be accelerated to Vy (or Vbroc, depending on whether your RFM is American or European!) and climbed to MSA.
Perro Rojo,
As pointed out, the TDP is an absolute after which the aircraft must be flown away in the event of the failure of a critical power unit if you are operating from a heliport where the accel/stop distance for a given airspeed is critical. For example, you may be required to operate from a 1200 metre heliport and in order to operate Class 1 you have to restrict your take off weight for a given temperature because of this. If you were to have an engine failure after TDP and decide to reject, then have any of your passengers suffer injury, the lawyers would indeed be after you and have a good case. As you say, if you have a long runway ahead, then it's your call as to whether to reject, but as I say, I've seen a lot of pilots mess it up when the runway isn't quite so long, or they have commenced a take off from a point other than the threshold where they have no idea exactly what distance they now actually have available.
Remember also that TDP refers only to engine problems in multi-engined helicopters. It's entirely single engine performance thing after one has failed and has nothing to do with gearbox, tail rotor or other non-engine-related problems. The other point to bear in mind is that even if operating class 2, you are allowed to damage the aircraft when you land, but if you injure a third party outside, then the lawyers will also be after you

[JimL]

Munk,

Continuously repeating a statement will not make it any more correct – TDP is far more complex than you make it out to be; it has to conform to the text of AC 29.55 (a)(2):

(2) The definition of the TDP is based on a minimum required total energy concept. A specific minimum combination of kinetic energy (airspeed) and potential energy (height) should be attained at the TDP to ensure that a continued takeoff can be accomplished following the complete failure of one engine. In § 29.55(b), TDP is required to be defined by no more than two parameters. When using a single parameter such as time, height, or airspeed as a method of identifying the TDP, the identification must be combined with a precisely defined takeoff path and crew procedure to provide the required equivalent level of safety. In addition, it should be demonstrated that the pilot technique used during the takeoff sequence is easily repeatable and consistently produces the required energy (i.e., airspeed and height combination) when the TDP time, height, or airspeed is attained. This condition should be verified during the flight test program.

In the underlined text you will see that, in the absence of kinetic energy (speed) there must be sufficient potential energy (height) alone to assure the ‘continued take-off’. There are potential complications with vertical procedures because some helicopters which sustain a critical failure before or at the TDP enter into an accelerative descending flight mode – it is therefore important that the exact procedure is employed when carrying out a rejected take-off (with some helicopters, this requires that, during the descent to land, the collective is raised and held at the FADEC 30 second rating).

In fact the TDP satisfies two main requirements; with a power-unit-failure recognised at the TDP it is: (a) the last point from where a rejected take-off can be carried out (in horizontal and vertical extent); and (b) the first point from where a continued take-off can be achieved.

In the continued take-off to the take-off distance (TODRH), all obstacles (including the take-off surface) must be avoided by a specified margin. To comply with PC1, the take-off mass must also be such that, from the TODRH, a sufficient climb gradient is achieved to ensure that all obstacles are missed (or avoided) by a defined margin (greater for IFR than VFR) in the take-off flight path.

To be able to achieve the continued take-off, the location of the TDP must be established sufficiently accurately to ensure that all succeeding points on the take-off flight path can be established in three dimensions. Obviously, the critical failure will be one at one-second before the TDP (or whatever the intervention time is establish to be); I would suggest that the probability of a failure precisely at that point is Extremely Remote and so margins will almost always be better than those planned.

If it is not expected that the Category A take-off will be flown as written (as discussed in some of the posts above) alternative procedures such as PC2 should be considered. If flying a PC2 procedure, alternative terms for decision/committal points should be used to ensure that all members of the crew are in agreement with the procedure/consequences should an engine failure occur. Following JAR-OPS and for offshore, they could be Rotation Point (RP) and Committal Point (CP). These calls should be part of the briefing routine in accordance with SOPs.

Jim

[Brom]

Flungdung,
Whilst I do agree with you re the definitions, consider the following...
The scenario...
On an IFR departure with cloudbase between 50 and 100ft, vis 150 -200 metres and the nearest single engine diversion almost an hour away (all legal for North Sea Cat A ops).
The problem...
Just after TDP, but before going into cloud, with the runway ahead of you still visible, you have an engine fire warning, would you continue the take-off, or put it back on the ground?
I'm sure I know what I would do. If I could get it back on the ground that's where it would go regardless of any 'legal definitions'.
Brom.

[rotordude]

On an IFR departure with cloudbase between 50 and 100ft, vis 150 -200 metres and the nearest single engine diversion almost an hour away (all legal for North Sea Cat A ops).
The problem...
Just after TDP, but before going into cloud, with the runway ahead of you still visible, you have an engine fire warning, would you continue the take-off, or put it back on the ground?

Not trying to make an argument about what´s right or wrong here, but there is a couple of things one should be aware of during a low visibility TO:

1. Determining the runway available after TDP is really difficult under these circumstances. Be prepared for an overrun.
2. "Ballooning" the AC, while trying to decelerate, might leave you in a situation where you end up IMC with 10-20 deg. nose up. This time with no fly away capability.

[212man]

I concur with the last two posts, but would add that the scenario you describe should no longer be described as a rejected take off: it is now an emergency descent from a climb, a separate thing entirely. Understanding this is key. As has been pointed out, it is not a manouevre that is trained for, is familiar, or has any graphs associated with. It's not wrong per se, just don't consider it a rejected take off, and be prepared for surprises.

[Munk]

Appreciate the detailed briefings from all regarding definition of TDP/CDP – should have revised the facts rather than rely on isolated experience. Currently, on the type I fly, the CDP, which is time based, coincides with ETL.
Couple of questions though:
212man: I thought that TDP was time based precisely because of the lack of accurate IAS values at low airspeeds. What types have TDPs which are IAS based?
Brom and 212man: Have you, or anyone else used a TDP equating to 70kts, or 60kts, or even 50kts?
In these cases, a reject just prior to TDP would be interesting.
And, soggyboxers: You must have observed during your training/checking on short runways that quite a few pilots negate the parameters of their (calculated) TDP by, for example, reducing the effect of the flare during a reject by over leveling the aircraft, or flaring insufficiently etc etc. How do you ensure consistency in this area?
And, finally, flungdung: Your dictionary definition uses the noun. Common courtesy dictates that I should retract a statement when proved wrong. Doesn’t mean that I have to….

[212man]

212man: I thought that TDP was time based precisely because of the lack of accurate IAS values at low airspeeds. What types have TDPs which are IAS based?

Civil IAS gauges typically are invalid below 25 kts and on some older types are inaccurate below 40 kts. That's without introducing crosswinds, where the errors are compounded. So, yes, on some types where a low TDP airspeed is used, the method of determining that point is to count down. That relies on a given acceleration and an accurate time, so is inherently inaccurate in itself, but seems to be "close enough for government work!"

Types that have a TDP based on IAS? Well, I can personally vouch for the AS332, S-76, B212, EC-155, S-61 and S-92, and I know other types do too.

Brom and 212man: Have you, or anyone else used a TDP equating to 70kts, or 60kts, or even 50kts?

On the EC-155 the TDP range was from 30 KIAS to 70 KIAS, resulting in Vtoss values from 40-80 KIAS. 60 KIAS was the change over point from first segment to second segment limiting, i.e. there was no further payload benefit from higher 70 or 80 KIAS Vtoss, but it left your decision till later. Precisely for the reasons stated (going IMC etc.) On the S-92 the TDP range is 30 - 55 KIAS with Vtoss values of 45-70 KIAS.

In these cases, a reject just prior to TDP would be interesting.
And, soggyboxers: You must have observed during your training/checking on short runways that quite a few pilots negate the parameters of their (calculated) TDP by, for example, reducing the effect of the flare during a reject by over leveling the aircraft, or flaring insufficiently etc etc. How do you ensure consistency in this area?

I have always taught maximum performance reject technique (as described in the RFM) so that it is the instinctive first reaction if the failure occurs for real. If, you decide that with 2000m of runway left you'd like to back off the flare, or not droop the guts out of it, that fine, on the day. But, if you have always been taught the gentle, run it on with 30 kts type technique, then come the day you really are RTDRH limited (short runway, big trees at the end) then you don't want to be learning it as you go along (though the big trees will help the learning!) Same applies for landing.

(No, LDP is not the point of loss of ETL, either )

[tistisnot]

Gawd lavverdacks ..... no wonder the client complains of gobbledegook if we can't get this sorted amongst ourselves!!

I may be a stoopid line pilot, and you can call me ignorant of my professional duties, but there is somewhere when I call committed and before or after that I will carry out what was briefed - land/reject or continue/ go round.

[Mars]

OK Brom - I'm hooked; what would you do if you had the same emergency just as you entered cloud?

Have you ever discussed this in a CRM session?

Mars

[Max_Chat]

tistisnot, I agree with you that a committed (to land/fly away) is a great idea. Cat A Ops may say you must be assured of a given flight condition if OIE, but if all on board are aware of your intentions at a given point then all is clear.

As far as when TDP/LDP occurs, and what you do, differs with type so cut your cloth accordingly and the Lawyers will be no more well off than they are now. Win Win I think.

[Swamp76]

Flungdung,
Whilst I do agree with you re the definitions, consider the following...

The scenario...
On an IFR departure with cloudbase between 50 and 100ft, vis 150 -200 metres and the nearest single engine diversion almost an hour away (all legal for North Sea Cat A ops).
The problem...
Just after TDP, but before going into cloud, with the runway ahead of you still visible, you have an engine fire warning, would you continue the take-off, or put it back on the ground?
I'm sure I know what I would do. If I could get it back on the ground that's where it would go regardless of any 'legal definitions'.
Brom.

All this discussion only applies to OEI. All other issues, problems, emergencies are a seperate issue.

This what happens when we use airplane logic/regulations and apply it to helicopters.