Case study
Aircraft Leading-Edge Ice Stall Speed Margin Case Study
Leading-edge ice case study for CLmax reduction, stall speed increase, drag penalty, go-around margin, inspection evidence, and release criteria.
Leading-edge ice can reduce aircraft margin long before the wing looks dramatically contaminated. A rough ridge or distributed accretion near the leading edge changes the pressure distribution, trips the boundary layer, promotes early separation, reduces maximum lift coefficient, increases drag, and can change stall warning behaviour. The engineering decision is not whether ice is visible. It is whether the actual configuration still supports the required speed, manoeuvre, climb and release margins.
This case study follows a turboprop flight-test and operations review after residual leading-edge ice is found during an approach in icing conditions. The aircraft remains controllable, but airspeed targets, stall margin and missed-approach climb assumptions were based on a clean-wing configuration. The case is simplified for engineering learning and is not flight guidance. Real decisions must follow the approved flight manual, icing limitations, certification basis, operator procedures, maintenance inspection criteria and pilot authority.
Case Context
The aircraft is returning from a test support mission after operating in visible moisture near freezing conditions. Anti-ice was selected, but post-flight photographs and a line inspection show a shallow rough ridge on part of the wing leading edge. The crew reported slightly higher power for the same approach speed and a more abrupt buffet onset during a handling check before the final approach.
The central engineering question is:
Can the aircraft be treated as clean for approach and missed-approach performance, or does the observed leading-edge contamination invalidate the clean-wing stall and climb margins?
The answer requires a quantitative margin check and a release decision, not only a visual statement that the ice is small.
Simplified Aircraft and Test Data
Use the following representative data.
| Quantity | Symbol | Value |
|---|---|---|
| aircraft weight during approach | W | 68{,}000\ \text{N} |
| wing reference area | S | 30.2\ \text{m}^2 |
| local air density | \rho | 1.02\ \text{kg/m}^3 |
| clean landing-configuration maximum lift coefficient | C_{L,max,clean} | 2.10 |
| estimated iced maximum lift coefficient | C_{L,max,ice} | 1.55 |
| normal clean approach target | V_{app,clean} | 116\ \text{kt} |
| candidate contaminated-wing approach target | V_{app,ice} | 135\ \text{kt} |
| drag coefficient at approach lift, clean | C_{D,clean} | 0.120 |
| drag coefficient at approach lift, iced | C_{D,ice} | 0.180 |
| installed thrust available for missed approach with anti-ice | T_{avail} | 15.6\ \text{kN} |
| minimum missed-approach climb-gradient screen | \gamma_{req} | 3.3\% |
The iced C_{L,max} and drag coefficient are simplified engineering estimates based on a conservative ice-contamination penalty. Real programs need approved performance data, flight-test evidence, wind-tunnel or icing-tunnel data, computational evidence where justified, and operational limitations.
Field Evidence
The evidence is mixed but consistent with a contaminated-wing margin problem.
| Evidence | Engineering interpretation |
|---|---|
| leading-edge roughness visible on the outboard wing | a small shape change can affect separation and stall progression |
| higher torque required to maintain the same approach speed | drag penalty is plausible |
| buffet onset reported earlier than expected | C_{L,max} may be reduced |
| no air-data blockage fault recorded | this is not primarily a pitot-static case |
| deicing boot cycle completed normally | successful actuation does not prove a clean aerodynamic surface |
| aircraft remained controllable | controllability is not proof of adequate stall or climb margin |
The review should therefore treat the aircraft as aerodynamically different from the clean configuration until evidence proves otherwise.
Step 1: Calculate Clean Stall Speed
For steady level stall-speed screening:
Use the clean maximum lift coefficient:
Convert to knots using 1\ \text{m/s}=1.944\ \text{kt}:
The normal clean approach target is approximately:
This matches the planned clean approach speed.
Engineering Comment
The clean calculation is internally consistent. That does not make it valid after ice contamination. The key input is C_{L,max}, and the leading edge is one of the most sensitive parts of the wing for maximum-lift behaviour.
Step 2: Calculate Iced Stall Speed
Use the contaminated-wing estimate:
Then:
Convert:
Stall speed increase:
So the estimated stall speed increases by:
The approach speed needed for a 1.3V_S screen becomes:
Engineering Comment
The clean 116\ \text{kt} approach speed is no longer a 1.3V_S target. Relative to the iced stall speed:
That margin is much smaller than intended. The aircraft may still be flying at 116\ \text{kt}, but the planned stall margin has been lost.
Step 3: Include Load Factor Margin
Stall speed rises with load factor:
For a 30^\circ bank:
Iced turning stall speed:
At the clean approach target:
Engineering Comment
This is a weak margin for any low-altitude manoeuvre, gust, speed decay, pilot workload, autopilot mode transition or unexpected bank angle. The case should not be closed by saying the aircraft is above wings-level stall speed. The relevant condition includes manoeuvre margin and the contaminated configuration.
Step 4: Estimate Drag Penalty at the Contaminated-Wing Speed
Use the contaminated-wing approach target:
Clean drag at this speed and configuration:
Iced drag:
Additional drag:
Engineering Comment
Increasing approach speed to regain stall margin is not free. Dynamic pressure rises, and ice increases drag coefficient. A contaminated-wing speed target must therefore be checked against thrust, missed-approach climb, configuration limits, runway length, handling and flight-manual procedures.
Step 5: Check Missed-Approach Climb Margin
Use a simplified climb-gradient screen:
With anti-ice operating, installed thrust available is:
Using the iced drag estimate:
The minimum screen is:
The simplified margin is therefore slightly negative:
Engineering Comment
This result does not certify failure of the actual aircraft. It does show that the found condition is not a casual operational detail. The contaminated-wing case consumes both stall margin and climb margin. Release should require approved data or a conservative restriction, not a clean-wing calculation with an informal speed additive.
Step 6: Engineering Decision
The aircraft should not be released using clean-wing approach and missed-approach assumptions. The engineering decision is:
Hold normal release for the clean performance basis, treat the aircraft as contaminated until inspected and cleared, use only approved icing and contaminated-configuration procedures, and require evidence that stall margin, climb margin, anti-ice function and surface condition match the release basis.
Immediate actions:
- document the ice shape, location and extent before removal if safe to do so;
- compare the condition with approved icing limitations and aircraft maintenance criteria;
- verify anti-ice or deicing system function, indications and cycle timing;
- recalculate stall margin with the applicable contaminated C_{L,max} or approved speed schedule;
- recalculate missed-approach climb with anti-ice thrust, contaminated drag and actual weight;
- restrict manoeuvre, approach or dispatch assumptions if approved data are not available;
- inspect leading edges, probes, drains, boots, sensors and control surfaces before next release;
- record the decision basis in the flight-test or operations review package.
Failure Modes and Controls
| Failure mode | Effect | Control weakness | Stronger control |
|---|---|---|---|
| clean C_{L,max} used after leading-edge contamination | stall speed underestimated | visual ice noted but not tied to performance model | require configuration status in the performance worksheet |
| speed additive applied without climb check | stall margin improves but thrust margin may fail | speed and climb reviewed separately | couple stall, drag and climb screens |
| anti-ice status assumed from switch position | residual contamination missed | no surface evidence requirement | include post-cycle inspection or approved sensor evidence |
| stall warning expected at clean threshold | warning may not provide intended margin | warning logic not adjusted for contamination | validate warning and protection logic for icing assumptions |
| small bank angle ignored during approach | turning stall margin underestimated | wings-level check only | include load-factor screen for likely manoeuvre |
A qualitative RPN screen shows the risk concentration.
| Risk item | Severity | Occurrence | Detection | RPN |
|---|---|---|---|---|
| clean stall speed used with residual leading-edge ice | 9 | 3 | 5 | 135 |
| contaminated drag excluded from missed-approach climb | 8 | 3 | 6 | 144 |
| anti-ice system considered successful without surface evidence | 7 | 4 | 5 | 140 |
The exact scores are less important than the controls they trigger: configuration discipline, evidence of surface condition, coupled performance calculations and conservative release criteria.
Release Criteria
Release should require the aerodynamic configuration and the calculation basis to agree.
| Criterion | Required evidence |
|---|---|
| surface condition | leading edges and relevant lifting surfaces are clean, or the approved contaminated condition is explicitly used |
| stall speed basis | C_{L,max}, configuration, weight and density match the calculation |
| approach speed | speed target satisfies the required stall margin for the actual configuration |
| manoeuvre margin | likely bank angle and gust allowances are included where required |
| drag penalty | contaminated drag is included in climb, go-around or missed-approach checks |
| thrust basis | anti-ice, bleed, engine setting and installation effects match the performance data |
| warning/protection | stall warning, angle-of-attack logic or envelope protection remains valid for the allowed condition |
| inspection closeout | maintenance or flight-test evidence confirms the surface and system state before release |
The release question is not “is there enough lift right now?” It is whether the aircraft has the required margin through the expected operating sequence with the actual aerodynamic surface.
Transferable Lessons
Leading-edge ice is a configuration change. It should be treated like a change to C_{L,max}, drag, stall progression, warning behaviour and climb performance.
The practical diagnostic sequence is:
- freeze the configuration and document the contamination;
- replace clean C_{L,max} with an approved or conservative contaminated value;
- compute clean and contaminated stall speeds;
- include load factor for likely manoeuvres;
- compute drag penalty at the revised speed;
- check missed-approach or climb margin with anti-ice thrust assumptions;
- release only when surface condition, performance data and operating limits agree.
This case is distinct from a general stall-speed exercise. The technical lesson is the engineering decision around configuration evidence: a small leading-edge change can invalidate clean aerodynamic assumptions, and a speed correction must be checked against drag and thrust margins before it can support release.