Are stealth aircraft really invisible? Technical analysis of fighter aircraft stealth, its physical limitations, and conditions for effectiveness.

The practical limitations of stealth

The stealth of fighter aircraft is a concept that is often misunderstood. The general public, influenced by marketing and media hype, associates the term with a kind of absolute invisibility, as if a stealth aircraft could disappear from radar, infrared sensors, or optical systems. This fantasy, perpetuated by certain manufacturers, does not stand up to rigorous analysis of the laws of physics and operational realities.

Aircraft described as stealth—such as the F-35 Lightning II, the B-2 Spirit, and the Chengdu J-20—are not invisible. They are designed to reduce their signature in different bands of the electromagnetic spectrum. This makes them harder to detect, delays their identification, and makes them more difficult to engage. However, they remain visible under certain conditions, to certain sensors, or at certain distances.

Stealth is therefore a reduction in the probability of detection, never complete disappearance. It relies on compromises between aerodynamic design, absorbent materials, infrared treatment, and tactical discipline. To say that an aircraft is “invisible” is therefore misleading. The correct term is low observability. This report examines in detail under what conditions a stealth aircraft can actually escape detection, and under what other conditions it becomes vulnerable.

Operation based on reducing the radar equivalent surface area

The first dimension of fighter aircraft stealth is the radar signature, quantified by the radar equivalent surface area (RESA). This represents an object’s ability to reflect a radar wave. A conventional aircraft such as the Su-30 has an RCS of around 4 to 10 m². An F-35, thanks to its angular design and absorbent materials, has an RCS of less than 0.005 m² on the frontal axis.

This reduction does not mean invisibility. A sufficiently powerful radar, or one positioned at a lateral angle, will pick up part of the signal. In addition, the radar’s transmission frequency plays a major role: VHF or UHF band radars, although less accurate, are more effective against stealth shapes optimized for the X band.

Multistatic sensors, distributed across several platforms, also enhance detection capabilities. A stealth aircraft may go undetected by conventional monostatic radar, but it will be more easily spotted if several antennas triangulate its position from weak echoes.

Another limitation is that absorbent materials, such as RAM (Radar Absorbent Material) composites, degrade over time, due to weather and minor impacts. Their effectiveness therefore depends on constant and rigorous maintenance. An F-35 or B-2 with damaged paint will see its stealth capabilities rapidly decline.

In short, reduced SER works mainly against certain radars, at a given angle, and at a defined distance. It does not make the aircraft undetectable, but reduces the enemy’s acquisition range and reaction time.

Vulnerability to infrared and passive sensors

Another critical area where fighter aircraft stealth shows its limitations is infrared. All aircraft generate a thermal signature due to the heat from their engines, the heating of their airframe by friction with the air, and afterburning. Modern optronic sensors, such as IRST (InfraRed Search and Track), detect these heat sources at long range without active emission.

For example, the Pirate sensor on the European Typhoon or the OLS-35 on the Su-35 can locate an F-35 or B-2 at over 50 km, or even further in good atmospheric conditions. At high altitudes, in cold, dry air, thermal contrasts are marked, which favors passive observation.

Stealth aircraft do not yet have an active system for cooling their exhaust gases efficiently without sacrificing thrust. Even in economy mode, turbojet engines emit a significant signature that is visible on infrared bands. The enemy can then engage without using its radar, which partially neutralizes stealth.

In addition, optronic systems combine infrared cameras and high-definition optics capable of detecting visual anomalies (shapes, condensation trails, reflections). Over calm seas or clear skies, these signals are sufficient to launch an interception or visual identification.

Thus, thermal stealth remains a weakness, despite efforts such as nozzle alignment, thermal shielding, and the use of low-emissivity materials. A stealth aircraft remains visible in infrared. It cannot therefore rely solely on its radar SER to avoid detection.

Strong tactical and logistical dependence to maintain the element of surprise

Even if a stealth aircraft is designed to minimize its signature, it is not autonomous in its ability to remain undetected. It depends on a whole doctrine of use, the combat environment, and its own electromagnetic emissions.

An aircraft that actively uses its radar, data links, or radio transmissions betrays its position. This is the paradox of stealth: to remain discreet, the aircraft must often sacrifice certain functions. This requires the use of an external network of sensors (AWACS, satellites, drones) to provide it with a tactical picture without emitting any signals.

This also assumes that the enemy does not have overly dense radar coverage or air defense spread across multiple vectors. In Ukraine, for example, several low-signature American drones were detected, tracked, and sometimes shot down by integrated network sensors. Combined ground-to-air radar integration can partially negate the benefits of stealth.

Flying a stealth fighter also requires meticulous planning: altitude, speed, weather, flight paths, mission times. A single error in trajectory can expose the aircraft to a radar beam or a thermal zone. Stealth is therefore not a permanent intrinsic property. It is a transient effect that depends on context and tactical discipline.

Finally, adversaries are developing countermeasures: AI for processing radar noise, passive radars that exploit reflected civilian signals, and wide-area weapons that do not aim precisely but saturate an area. Stealth aircraft must therefore constantly adapt their doctrine and can only operate effectively in an environment that is technologically controlled by their allies.

A strategic tool, but far from the promised invisibility

The idea that a stealth aircraft is invisible is therefore a misnomer, perpetuated for political or commercial reasons. In practice, stealth is a complex, expensive tool with limited effectiveness. It delays detection, complicates pursuit, and makes firing more difficult, but never completely eliminates the risk.

The United States has invested more than €1.3 trillion in the development and deployment of the F-35. Part of this sum covers technologies aimed at delaying detection as long as possible, not avoiding it altogether. China and Russia are following the same path, with less industrial rigor but similar objectives.

A stealth fighter jet flight is therefore not an invisible flight. It is a flight whose purpose is to cross defended areas without giving the enemy time to act. It is not a guarantee of survival, but a tool for strategic penetration, first strike, or precision escort.

A stealth aircraft is detectable. Less easily, in fewer cases, but it remains vulnerable. Advances in passive defenses, artificial intelligence, and long-range radar will gradually reduce the benefits of passive stealth. The next step will be active stealth, dynamic onboard jamming, or the use of decoy drones.