The Flight Management System (FMS) is the computational backbone of modern transport-category and advanced general aviation aircraft. Pilots interact with it almost entirely through the Control Display Unit (CDU), a keyboard-and-screen interface that serves as the command terminal for routing, performance, and navigation data. At the heart of this interface is the scratchpad — a single line at the bottom of the CDU display where all data entry begins before it is transferred into the active flight plan or performance pages. Understanding exactly how that scratchpad functions, what its error and advisory messages mean, and how the system validates data before accepting it, is not just a test topic — it directly affects whether the FMS executes the flight you intend.
This article treats the scratchpad as a system in its own right, explaining its architecture, the categories of errors it produces, the logic the FMS uses to validate data, and the practical habits that prevent automation surprises in the flight deck.
The Scratchpad: Architecture and Role
Every keystroke on the CDU alphanumeric keyboard is echoed into the scratchpad line before any data is committed to the system. This design creates a buffer — a staging area — where the pilot can review an entry and correct it before pressing the appropriate Line Select Key (LSK) to transfer the data into its target field. The scratchpad therefore acts as both an input queue and a message display area.
When no data is being entered, the scratchpad can display one of two categories of text: a pilot-entered string awaiting transfer, or an FMS-generated message. These two functions share the same physical line, which is an important source of confusion. An FMS message sitting in the scratchpad must be acknowledged (usually by pressing the CLR key) before new data entry can begin. Failing to recognize this means keystrokes are lost or appended to the message text rather than starting a fresh entry — a common CDU error in high-workload environments.
Categories of Scratchpad Messages
FMS manufacturers use slightly different terminology, but FAA guidance and training doctrine generally recognize two broad classes of scratchpad output beyond normal data strings:
Error Messages
Error messages appear when the FMS rejects an entry outright. The data did not pass validation and was not written into any field. Common examples include:
- INVALID ENTRY — The format of the data does not match what the field expects. For example, typing a waypoint identifier that does not exist in the navigation database, entering a flight level using an incorrect format, or providing a wind entry without both direction and speed components will trigger this message. The entry is discarded entirely.
- NOT IN DATABASE — A specific subset of invalid entry: the identifier or procedure name is syntactically correct but cannot be found in the currently loaded navigation database. This frequently occurs with pilot-defined waypoints that were not properly created, outdated database cycles, or regional databases that do not include all global identifiers.
- ENTRY OUT OF RANGE — The value is correctly formatted but exceeds the acceptable numeric limits for that field. An airspeed entered above the aircraft's maximum operating speed, a cost index outside the computed range, or an altitude beyond the aircraft's certified ceiling can produce this error.
- UNABLE NEXT ALT or similar performance errors — These arise not from a format problem but from a performance constraint the FMS calculates cannot be met. The entry is rejected because the system's internal performance model determines it is physically impossible given current weight, thrust, or atmospheric data.
Advisory Messages
Advisory messages do not necessarily reject an entry but alert the crew to a condition requiring attention. Examples include CHECK HOLD SPEED, DRAG REQUIRED, or FUEL DISAGREE. These messages are informational; the pilot must clear them to continue data entry, but they signal that a computed value differs from expectations or that a cross-check with raw instruments is warranted. Ignoring advisory messages and simply clearing them without understanding their meaning is one of the most common — and dangerous — automation complacency errors.
Data Validation Logic
Before accepting any entry, the FMS runs it through a multi-layer validation process. Understanding these layers helps pilots anticipate what will and will not be accepted:
Syntax Validation
The first check confirms that the entry matches the expected format for the target field. Waypoint identifiers are checked for correct alphanumeric length (typically 2–5 characters). Coordinates must follow the correct latitude/longitude format. Altitudes entered on an altitude constraint field must be numeric and, where required, include a flight level or feet designation consistent with the aircraft's configuration. A syntax failure produces an INVALID ENTRY message immediately.
Database Validation
Once the format passes, the FMS searches its onboard navigation database — which is updated in 28-day AIRAC cycles per ICAO standards — for a matching record. If no match is found, NOT IN DATABASE is returned. Pilots should be aware that the database in use must be current for the period of flight; navigation database currency requirements for IFR operations are established through FAA Advisory Circular 90-100A, TSO/equipment approval requirements, and airline operations specifications, rather than 14 CFR 91.503 (which addresses flying equipment and operating information for large and turbine-powered multiengine airplanes, not database currency). Using an expired database may result in procedures, waypoints, or airways simply not being present, generating database errors that would not occur with a current cycle.
Range and Reasonableness Validation
Numeric fields are checked against hard limits (absolute maximums built into the software) and sometimes soft limits (performance-based bounds that can vary with conditions). A hard-limit rejection produces ENTRY OUT OF RANGE. Some systems also perform a reasonableness check comparing new data against already-entered data — for example, checking whether a manually entered wind is grossly inconsistent with the forecast winds already loaded for other waypoints, or whether a manually entered gross weight is inconsistent with the fuel quantity sensor data.
Constraint Feasibility Validation
This is the most computationally intensive layer. After accepting a value syntactically and finding it in the database, the FMS integrates it into its performance model. If complying with a new altitude or speed constraint would require thrust or drag beyond the aircraft's modeled capability, the FMS may reject the entry or flag it with an advisory. This layer protects against impossible clearances being silently accepted and then missed because the FMS simply never reached the constraint altitude.
Why Scratchpad Error Management Matters
A misunderstood or hastily cleared scratchpad message has contributed to numerous incidents in transport aviation. The core risk is this: the crew believes data has been accepted and the FMS is executing the intended routing or performance target, when in fact an error message rejected the entry and the previous (possibly outdated) data remains active. This gap between what the crew thinks the FMS is doing and what it is actually doing is the defining hazard of automation complacency, described extensively in FAA Risk Management Handbook (FAA-H-8083-2) and the broader body of Crew Resource Management doctrine.
Specific real-world consequences include: wrong altitudes on departure procedures because a constraint was rejected and the default profile was retained; incorrect destination fuel planning because a gross weight entry failed validation and the FMS continued using an old value; and route deviations because a waypoint identifier triggered NOT IN DATABASE and was silently bypassed without crew awareness.
Key Numbers and Rules
- 28-day AIRAC cycle — the standard interval for navigation database updates; currency is operationally and, for many operations, regulatory required.
- CLR key — clears the scratchpad one character at a time if data is present; clears an FMS message if the scratchpad is otherwise empty. Pressing CLR when a message is displayed acknowledges and removes it.
- DEL key — on many FMS platforms, transfers the DELETE function to an LSK field, removing a waypoint or constraint rather than clearing the scratchpad; crews must distinguish CLR from DEL to avoid unintended deletions.
- Dual CDU cross-check — airline SOPs typically require the pilot monitoring (PM) to verify critical entries on their own CDU after the pilot flying (PF) completes data entry, catching errors before they propagate into the active flight plan.
- After every data entry — best practice requires a read-back of the field as displayed on the CDU (not as typed in the scratchpad) to confirm the FMS accepted the value the crew intended.
Common Test Traps
- Confusing error messages with advisory messages. Test questions may describe a scenario where an advisory appears and ask what action is required. The answer is to understand and evaluate the advisory — not merely to clear it. An error message means the entry was rejected; an advisory means the entry may have been accepted but a condition warrants attention.
- Assuming a cleared message means an accepted entry. Pressing CLR removes the message from the scratchpad display but does not confirm the entry was written. Always verify the target field shows the intended value after transferring data.
- Database currency traps. Questions may describe NOT IN DATABASE errors and ask the cause. The correct answer often involves an expired AIRAC cycle or a procedure added after the database was compiled — not pilot format error.
- CLR vs. DEL confusion. The exam may test knowledge of which key removes a constraint from a flight plan field versus which key clears the scratchpad. These have very different effects on the active route.
- Performance rejection vs. format rejection. ENTRY OUT OF RANGE means a correctly formatted number exceeded allowable bounds. INVALID ENTRY means the format itself was wrong. Knowing the distinction matters for diagnosing why an entry was rejected and determining the correct corrective action.
Mastering the FMS scratchpad is ultimately about maintaining an accurate mental model of what the automation is doing at all times. Every error message is the FMS communicating a boundary it could not cross; every advisory is a flag requesting pilot judgment. Treating these messages as interruptions to be dismissed rather than information to be understood is the root cause of the automation-related errors that recur in incident reports across the industry. The CDU scratchpad, properly understood, is not a source of frustration — it is one of the most honest feedback mechanisms in the flight deck.