Intercept Geometry: Table of Contents
- Part I: Introduction
- Part II: Definitions
- Part III: Target Aspect & Lateral Separation
- Part IV: Modern Gameplans (P-825/17)
- Part V: P-825/17: DT, CT, Timeline
- Part VI: Modern Intercept Demo Videos (P-825/17)
- Part VII: 2000s Intercepts (P-825/02)
- Part VIII: Intercept Progression & Lead Collision (P-825/02)
- The procedure described in the P-825/02 is quite simple: CC → DT → CT.
- The goal is also different: it is 40,000ft of LS at 10nm for modern procedures, here is 20,000ft of LS.
- The DT is not a contingency manoeuvre but an actual part of the intercept since the LS is not adjusted prior to the 20k LS goal
The documentation available almost 20 years ago is somewhat similar to the P-825/17 discussed before. There are some differences, for example the aircraft used as reference for the documentation is the North American T-39N Sabreliner. This also affects the amount of turning room necessary for a conversion. In fact, the LS goal is now 20,000ft whereas before (or, better, nowadays, is 40,000ft). The fact that the documentation clearly writes that:
“we pushed the T-39N to the edge of the envelope […] the T-39N will need more lateral displacement than high performance fighters“
raises the question of whether the LS goal depends on the training aircraft used (T-45 vs T-39) or the doctrine changed during the years.
Similarly to the modern documentation, the P-825/02 discuss the elementary Stern Conversion Turn. It does not suggest any gameplan however, reducing the flow to Collision Course into a Displacement Turn, necessary to create the room for the Counterturn. In this respect, it goes more deep into the topics of the Collision, Displacement Turn and Counterturn, using the b-scope as the primary tool and suggesting different formulas and procedure to calculate all the variables involved into a successful intercept: Target Aspect, Angle Off (ATA), Collision Course Corrections, and so on. The aim of this article is to briefly present such concepts (for more details, refer to the actual documentation).
Collision Bearing and other stories
Thanks to the work done on the TID AS, we know that the collision course can be visually established by just a glance at the TID in Aircraft Stabilized mode. On the other hand, there are more elegant ways to establish the value of the Collision Bearing (CB).
In the meantime, let’s refresh and expand some other relevant concepts discussed in the P-825/02.
On the other hand, knowing the relations that are valid in such scenario help the RIO to determine the values he needs as he can try or approximate such situation.
In case of a co-speed intercept, calculating the CB is quite simple:
When the Collision Course is established, ATA = CB.
Non co-speed Intercept
The non co-speed intercept is a topic not discussed in the documentation so far, so I put together a simple model to quantify how the CB in this scenario diverges from CBCOSPD. The conclusions of that study should help the RIO to understand what to expect and how much ΔCB changes depending on ΔV and Cut / DTG.
Unfortunately I have not found doctrinal and declassified documentation covering non co-speed intercepts so far, so we have to work with what we have plus a bit of ingenuity.
The amount of drift can be quantified as the amount of degrees of change in ATA per nautical mile of SR.
- Intercept: on CB, collision course is established.
- Visible: 0.5° azimuth change per nm of SR;
- Moderate: 1° azimuth change per nm of SR;
- Sharp: 2° azimuth change per nm of SR;
- Flat: 3° azimuth change per nm of SR.
Variation of ATA
Provided there is no change in bogey or fighter heading, angle off can be predicted by utilizing the following formula: As the range halves, the degrees the bogey is off Collision bearing will double.
Collision Course Corrections (CCC)
This formula is based on the simple fact that, when co-speed and on Collision, TA = ATA (but opposite in sign):
The image below clarifies the relation.
This formula can be immediately verified by applying the usual relation: CBCOSPD = Cut / 2. From this value, we can then determine the BB. In this example, the resulting BB matches the expected value.
The formula above is not new to us, I accidentally run into it in Part III, (Undiscussed Scenarios). Be careful though as the relation Cut = TA + ATA is not always applicable: TA and ATA must be, in fact, opposite in sign, otherwise the relation changes and Cut = |TA – ATA|. In other words, as long as the standard intercept triangle is applicable, the relation does not change.
The second formula is used to adjust the CC and correct the imprecisions of the GCI controller’s BB as they are less accurate than the radar scope information.
The idea is to calculate the ATA from the scope (we have the TID) and compare it with the Cut. The goal is making the ATA equal to the TA but opposite in sign.
Cut → ATA = ATA → CC
Quoting the P-825/02, p. 49:
This method is referred to as “bouncing the ball.” To determine the CCC, bounce the ball from cut to ATA, then bounce it in the same direction and distance to discover the CCC.
I usually find the explanations on the doctrinal docs too detailed or not detailed enough but in this case is simple and spot on so, rather than spending time re-drawing the scenario, I will simply take them from the P-825/02.
This formula can be computed directly from the previous: Cut = TA + ATA. Because CB = Cut / 2 when aircraft fly co-speed, we can resolve the equation for CB. Moreover, in such scenario TA = ATA but opposite in sign.
The Displacement Turn (DT) is used to create the turning room necessary to the fighter before turning to the bandit’s RQ.
The DT is conceptually different in this version of the doctrine: in the most recent documentation it was a contingency manoeuvre, used to compensate what de facto is a mistake. In this older version instead is part of the intercept itself.
Following extensive, and often dangerous, flight testing, we pushed the T-39N to the edge of the envelope and concluded that 20,000 feet (roughly 3 1/2 nm) is the ideal amount of turning room for the reattack intercept. As you can imagine, the T-39N will need more lateral displacement than high performance fighters.
This paragraph opens the question: what is the criteria behind the choice of creating 40,000 ft, 20,000 ft or other amounts of Lateral Separation? Is it the aircraft (T-45 vs T-39)? The desire to avoid flying hot (TA < 30) towards the target, to perhaps exploit its lack of SA? Or something else again?
As always when I discussed the procedures, feel free to adopt the range that fits better the module you fly and your group’s SOP.
Objectives and Outcomes
The Displacement Turn has three objectives:
- Break collision course;
- Set FFP to: gain, maintain or slow the rate of loss of LS as required;
- Establish the bandit at approximately the same distance from CB (post displacement) no matter the TA. This is done so the drift rate after displacement is constant for all TA situations.
Post DT, the fighter can be oriented in three ways. Recalling Part III, the possible ways are: Cut Into, Cut Away, Zero Cut.
- Cut Away: if the Target Aspect was producing less Lateral Separation than the goal (20,000 ft), the FFP should be pointing away from the BFP to increase the LS;
- Zero Cut: if the LS was already satisfying the goal, then the FFP should be parallel to BFP (Zero-Cut captures the LS).
Note: since aircraft does not turn instantaneously some LS can be lost during the turn. Therefore, the fighter nose should be pointed slightly away from BFP after the DT;
- Cut Into: if the LS was more than the 20,000 ft of LS goal, then the FFP should be pointing slightly towards the bandit. This reduces the LS at a slower pace than Collision.
- Determine the Target Aspect: the initial step. The P-825/02 suggests to use the relation Cut → ATA = TA but, if the contact is lost, use BR → BB = TA.
Note that these formulas have been discussed in the previous chapters of this series;
- Determine the correct Displacement Range: remembering the formula used to calculate the Lateral Separation (LS = SR * TA * 100), in order to achieve 20,000ft of LS, the DT should start at 10nm. Rule of thumb:
TA ≤ 20° ► displace at 10nm
TA ≥ 25° ► displace at 8nm
- Determine the correct Displacement Point (ATA): left or right of the fighter’s nose, depending on the calculated TA;
- Command Pilot to use the appropriate hard turn: “Right (or Left) hard for displacement.”
The documentation aims to place the bandit at approximately 25° from CB.
To help the RIO with the calculations, the P-825/02 suggests that, quoting:
“it is necessary that the weapons officer memorize the following table such that displacement points and ranges can be instantly recalled.“
|0°L||10 nm||50°L||0°R||10 nm||50°R|
|5°L||10 nm||45°L||5°R||10 nm||45°R|
|10°L||10 nm||40°L||10°R||10 nm||40°R|
|15°L||10 nm||35°L||15°R||10 nm||35°R|
|20°L||10 nm||30°L||20°R||10 nm||30°R|
|25°L||8 nm||25°L||25°R||8 nm||25°R|
|30°L||8 nm||20°L||30°R||8 nm||20°R|
|35°L||8 nm||15°L||35°R||8 nm||15°R|
|40°L||8 nm||10°L||40°R||8 nm||10°R|
|45°L||8 nm||5°L||45°R||8 nm||5°R|
|50°L||8 nm||DA||50°R||8 nm||DA|
|55°L||8 nm||5°R||55°R||8 nm||5°L|
|60°L||8 nm||10°R||60°R||8 nm||10°L|
|65°L||8 nm||15°R||65°R||8 nm||15°L|
|70°L||8 nm||20°R||70°R||8 nm||20°L|
DA = Dead Ahead
Tips to memorize the table:
- DT are normally 50°, if the bandit was previously on CB;
- DT are normally on the same side of the nose (scoper) as the direction of TA, except when TA > 50°;
- TA + Displacement Point = 50.
The documentations detail the effect of the geometry in Cut Into, Cut Away and Zero-Cut scenarios. These have been discussed here already so I will skip them at this time. However, there is a particular scenario that should be considered: the case of TA rounding to zero degrees. For example, TA 2R must be displaced to 50R ATA for two reasons:
- a turn to 50L ATA would cross in front of the bandit (the turn will make the fighter quite visible);
- recalling the LS formula, a turn towards the wrong ATA sign (e.g. 50L ATA) would result in a loss of a few thousands of feet, possibly affecting the next phase of the intercept.
Review of the DT Principles
To wrap it up, this is a list of the principles governing the Displacement Turn:
- The DT is used to gain, preserve or slow the loss of LS;
- The amount of LS available is a function of SR and TA;
- The amount of LS available is unaffected by the existence (or lack of) collision intercept.
- DT aims to reposition the bandit 25° away from CB;
- DT is not necessarily a 50° turn. The bandit is repositioned ~50° from the original ATA only if collision was present;
- When TA < 50°:
– Right TA is displaced to the right side of the scope;
– Left TA to the left side of the scope.
The following table is the result of the computation of the principles mentioned above:
DT on LS
Observation: Differences between 2000s and Modern DT
Modern Intercept doctrine sees the DT as a contingency manoeuvre. The “bulk” of the LS manipulation (gameplans) is performed from the Commit to the last 10nm. The documentation from the beginning of the new millennia instead relies on Collision Course (therefore marginal LS manipulation) and DT is the main means of manipulating the LS before the Counterturn.
The determination of DT is different as well (i.e. the P-825/17 uses the “Rule of 40“).
The Counterturn (CT) is the last part of the intercept; it is the manoeuvre that places the fighter into the bandit’s RQ. It is executed after the DT, which should have placed the target approximately 25° off CB. The target will therefore drift away (intercept drift) from the fighter’s nose. This outwards drift is countered by manoeuvring in the opposite direction, ergo inducing turn drift and thereby maintining the bandit at the desired ATA.
Note that the maximum drift rates occur when the bandit is on or close to the fighter’s nose.
The Drift has been discussed multiple times in the past so I will not spend more time here, since the concepts of Turn Drift and Intercept Drift apply no matter the avionics or the documentation.
However, the Drift is vert important when discussing the Counterturn. During such manoeuvre in fact, the bandit will drift on the scope and the goal of the CT is “drawing” an appropriate Drift Curve.
Ideal Drift Pattern and Counterturn Sequence
Counterturns can be divided in three main categories, depending on the available LS (and therefore from the TA and the SR).
Low Target Aspect (TA 0°-15°)
Low Target Aspect means that Lateral Separation must be created by turning away from the bandit (Cut-Away). The bandit should be held at a constant ATA to allow LS to be gained.
At some point the ATA cannot be held constant whilst in performing a standard turn (or less). This point should be near 180DTG in the CT. Past this position, the nose of the fighter should be pointing towards the BFP, effectively reducing the Lateral Separation. The fighter should, if necessary to maintain the ATA, harden the turn to a hard as poss turn.
Finally, when the bandit is, ideally, on the fighter’s nose at 90 DTG, the drift rate is at the highest and the fighter should then attempt to roll out in the bandit’s RQ (½ nm – 1½ nm from the bandit), in range for a SRM shot.
Medium Target Aspect (TA 20°-25°)
Since the LS already satisfies the goal of 20,000ft, the DT in this scenario does not change the position of the fighter relative to the bandit.
The initial drift is countered by an easy turn (the RIO should monitor and command to hold, ease or harden the turn). At 90 DTG, 2nm SR, the bandit should be on the nose. By turning Harder, the fighter aims to roll out at the bandit’s RQ, within ½ nm – 1½ nm from it.
High Target Aspect (TA > 30°)
In this scenario the LS is too much and the DT drives the fighter torwards the bandit. The initial drift is countered by a standard turn, rather than easy. The drift should be constant and inwards, and the bandit will be on the nose before the 90 DTG position. As usual, the objective is rolling out in the bandit’s RQ, within ½ nm – 1½ nm.
Counterturn Sequence Chart
As usual, the following table should be commit to memory, at least according to the doctrine:
|CT SEQUENCE AFTER DR|
|0°||10 nm||50°||Easy – Standard – Hard as possible|
|5°||10 nm||45°||Easy – Standard – Hard as possible|
|10°||10 nm||40°||Easy – Standard – Hard as possible|
|15°||10 nm||35°||Easy – Standard – Hard as possible|
|20°||10 nm||30°||Easy – Standard – Hard – Hard as possible|
|25°||8 nm||25°||Standard – Hard – Hard as possible|
|30°||8 nm||20°||Standard – Hard – Hard as possible|
|35°||8 nm||15°||Standard – Hard – Hard as possible|
|40°||8 nm||10°||Standard – Hard – Hard as possible|
|45°||8 nm||5°||Standard – Hard – Hard as possible|
|50°||8 nm||DA||Standard – Hard – Hard as possible (?)|
There are a couple of rules of thumb that can help memorizing the process:
- Similarly to the DT, 20° acts as the “threshold” between higher range actions;
- Understandably, the closer the fighter is to the bandit (lower SR), the harder the manoeuvre has to be.
“Hot” and “Cold” CT
This is a concept we have enountered a few times already and the difference between the two is intuitive: Hot refers to a low TA / LS situation whereas Cold refers to a high TA / LS situation. Usually, Cold is more desirable than Hot.
Applied to Counterturns, Hot and Cold allow us to differentiate two situations:
- Cold Counterturn: the result is the fighter lagging behind the bandit, due to the LS not reduced enough, and in the correct position (RQ) but perhaps even too far for a SRM employment. At this stage the solution is adding throttle to reduce the SR;
- Hot Counterturn: the result is the fighter crossing the BFP in front of the bandit (dangerous!) with great risk of overshooting. It happens when the bandit is brought to the fighter’s nose too quickly.
Displacement Turn Variations
Since a proper DT is critical to a proper CT, the following table can be used to recognized imperfect situations and adjust them:
|DT Problem||Type Drift||Correction|
|Overdisplace > 5°||Cold||Redisplace immediately|
|Overdisplace < 5°||Cold||Start Counterturn|
|Underdisplaced > 5°||Hot||Redisplace immediately|
|Underdisplaced < 5°||Hot||Left drift to proper ATA|
Recovering from errors in CT: Compass Recovery and RQ Drift Control
A wrong assessment of the parameters of the Displacement Turn and the Counterturn can result in errors such as the fighter overshooting the target. The following are two techniques that can be used to try and salvage the situation.
(Note: remember the radar limitations when using these techniques, especially ZDF.)
This situation can origin when the target is in the hot side for too long, the 90 DTG is not recognized properly and the bandit is placed on the nose too soon, when VC is still too high. The result is the bandit drifting off the scope. The recovery procedure is simple, but unfortunately not always reliable:
- Command a “Hard as possible” turn to a heading 30° beyond BH;
- Call “Lost contact” to the Controller
- Anticipate radar contact on the nose, drifting towards BH;
- If no contact is acquired after 5″, proceed to a standard turn towards BH.
Rear Quarter (RQ) Drift Control
The RQ Drift Control is another useful manoeuvre to salvage the intercept. It can be executed both with and without radar contact.
RQ Drift Control – Without Radar Contact
If the bandit is allowed to drift and go off the scope, the procedure to execute is simple. Note that this requires a Controller (the AI AWACS as nowhere near an acceptable substitute unfortunately).
- Increase the turn rate until 45 DTG;
- Call “Lost contact” to the Controller – simultaneously with Step #1;
- Listen for BB from the Controller;
- Steady up heading half-way between BB and BH, once in RQ (45 DTG).
The rationale behind turning between BH and BB rather than towards one of the two is simple: by turning between BH and BB, the fighter is is Lead Pursuit and thereby allows to reduce the range and control the drift.
When the Controller sends an updated BB, the fighter corrects again until BB = BH, which signals that FFP = BFP.
RQ Drift Control – With Radar Contact
Once set in the RQ with low VC, the fighter should keep the bandit on the nose. The drift is minimal and the only undesirable event is crossing the BFP, resulting in a marginal overshoot. This is known as “Fighter weave“.
This concludes the overview of the concepts related to the Intercepts in the early 2000s. Next stop is the Intercept Progression.
I plan to record a couple of demo videos as well, as I did for the Modern Intercept.